[AMDGPU] Add file headers for few files where it is missing.

[hsmhsm]

Summary

Added file headers for files which implement iterative lightweight scheduling
strategies. Which is basically an exercise which I undertook in order to get
used to LLVM development process.

[hsmhsm]

Hello CD,

I have added you too as a reviewer for this patch. Kind request to review it.

[vpykhtin]

LGTM, thanks.

llvm/lib/Target/AMDGPU/GCNIterativeScheduler.h

 //===- GCNIterativeScheduler.h - GCN Scheduler ------------------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
+///
+/// \file
+/// This file defines the class GCNIterativeScheduler, which uses an iterative
+/// approach to find a best schedule for GCN architecture. It basically makes
+/// use of various lightweight schedules, scores them, chooses best one based on
+/// their scores, and finally implements the chosen one.
+///
+//===----------------------------------------------------------------------===//
 
 #ifndef LLVM_LIB_TARGET_AMDGPU_GCNITERATIVESCHEDULER_H
 #define LLVM_LIB_TARGET_AMDGPU_GCNITERATIVESCHEDULER_H
 
 #include "GCNRegPressure.h"
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/CodeGen/MachineBasicBlock.h"
 #include "llvm/CodeGen/MachineScheduler.h"

llvm/lib/Target/AMDGPU/GCNIterativeScheduler.cpp

 //===- GCNIterativeScheduler.cpp ------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
+///
+/// \file
+/// This file implements the class GCNIterativeScheduler.
+///
+//===----------------------------------------------------------------------===//
 
 #include "GCNIterativeScheduler.h"
 #include "AMDGPUSubtarget.h"
 #include "GCNRegPressure.h"
 #include "GCNSchedStrategy.h"
 #include "SIMachineFunctionInfo.h"
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/ADT/STLExtras.h"

llvm/lib/Target/AMDGPU/GCNMinRegStrategy.cpp

 //===- GCNMinRegStrategy.cpp ----------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
+///
+/// \file
+/// This file defines and imlements the class GCNMinRegScheduler, which
+/// implements an experimental, simple scheduler whose main goal is to learn
+/// ways about consuming less possible registers for a region.
+///
+//===----------------------------------------------------------------------===//
 
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/ADT/SmallPtrSet.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/ilist_node.h"
 #include "llvm/ADT/simple_ilist.h"
 #include "llvm/CodeGen/ScheduleDAG.h"
 #include "llvm/Support/Allocator.h"

llvm/lib/Target/AMDGPU/GCNRegPressure.h

 //===- GCNRegPressure.h -----------------------------------------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
+///
+/// \file
+/// This file defines the GCNRegPressure class, which tracks registry pressure
+/// by bookkeeping number of SGPR/VGPRs used, weights for large SGPR/VGPRs. It
+/// also implements a compare function, which compares different register
+/// pressures, and declares one with max occupance as winner.
+///
+//===----------------------------------------------------------------------===//
 
 #ifndef LLVM_LIB_TARGET_AMDGPU_GCNREGPRESSURE_H
 #define LLVM_LIB_TARGET_AMDGPU_GCNREGPRESSURE_H
 
 #include "AMDGPUSubtarget.h"
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/CodeGen/LiveIntervals.h"
 #include "llvm/CodeGen/MachineBasicBlock.h"

llvm/lib/Target/AMDGPU/GCNRegPressure.cpp

 //===- GCNRegPressure.cpp -------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
+///
+/// \file
+/// This file implements the GCNRegPressure class.
+///
+//===----------------------------------------------------------------------===//
 
 #include "GCNRegPressure.h"
 #include "AMDGPUSubtarget.h"
 #include "SIRegisterInfo.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/CodeGen/LiveInterval.h"
 #include "llvm/CodeGen/LiveIntervals.h"
 #include "llvm/CodeGen/MachineInstr.h"