[AArch64][GlobalISel] Move GlobalISel source files to a dedicated subdir

[aemerson]

Summary

No real technical reason to move these, except to separate the GISel components for organization purposes. The tablegen files remain where they are.

I can imagine we're going to be creating more passes, or factoring out code into separate files in future, which would clutter things even more unless we split them.

[kristof.beyls]

I think I'd like the proposed organization better; but I only have a weak preference.
LGTM.

[paquette]

I'm fine with this

llvm/lib/Target/AArch64/AArch64Subtarget.cpp

 //===-- AArch64Subtarget.cpp - AArch64 Subtarget Information ----*- 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
 //
 //===----------------------------------------------------------------------===//
 //
 // This file implements the AArch64 specific subclass of TargetSubtarget.
 //
 //===----------------------------------------------------------------------===//
 
 #include "AArch64Subtarget.h"
 
 #include "AArch64.h"
-#include "AArch64CallLowering.h"
 #include "AArch64InstrInfo.h"
-#include "AArch64LegalizerInfo.h"
 #include "AArch64PBQPRegAlloc.h"
 #include "AArch64RegisterBankInfo.h"
 #include "AArch64TargetMachine.h"
+#include "GISel/AArch64CallLowering.h"
+#include "GISel/AArch64LegalizerInfo.h"
 #include "MCTargetDesc/AArch64AddressingModes.h"
 #include "llvm/CodeGen/GlobalISel/InstructionSelect.h"
 #include "llvm/CodeGen/MachineScheduler.h"
 #include "llvm/IR/GlobalValue.h"
 #include "llvm/Support/TargetParser.h"
 
 using namespace llvm;
 

llvm/lib/Target/AArch64/CMakeLists.txt

 tablegen(LLVM AArch64GenRegisterInfo.inc -gen-register-info)
 tablegen(LLVM AArch64GenSubtargetInfo.inc -gen-subtarget)
 tablegen(LLVM AArch64GenSystemOperands.inc -gen-searchable-tables)
 tablegen(LLVM AArch64GenExegesis.inc -gen-exegesis)
 
 add_public_tablegen_target(AArch64CommonTableGen)
 
 add_llvm_target(AArch64CodeGen
+  GISel/AArch64CallLowering.cpp
+  GISel/AArch64InstructionSelector.cpp
+  GISel/AArch64LegalizerInfo.cpp
+  GISel/AArch64PreLegalizerCombiner.cpp
+  GISel/AArch64PostLegalizerCombiner.cpp
   AArch64A57FPLoadBalancing.cpp
   AArch64AdvSIMDScalarPass.cpp
   AArch64AsmPrinter.cpp
   AArch64BranchTargets.cpp
   AArch64CallingConvention.cpp
-  AArch64CallLowering.cpp
   AArch64CleanupLocalDynamicTLSPass.cpp
   AArch64CollectLOH.cpp
   AArch64CondBrTuning.cpp
   AArch64ConditionalCompares.cpp
   AArch64DeadRegisterDefinitionsPass.cpp
   AArch64ExpandImm.cpp
   AArch64ExpandPseudoInsts.cpp
   AArch64FalkorHWPFFix.cpp
   AArch64FastISel.cpp
   AArch64A53Fix835769.cpp
   AArch64FrameLowering.cpp
   AArch64CompressJumpTables.cpp
   AArch64ConditionOptimizer.cpp
   AArch64RedundantCopyElimination.cpp
   AArch64ISelDAGToDAG.cpp
   AArch64ISelLowering.cpp
   AArch64InstrInfo.cpp
-  AArch64InstructionSelector.cpp
-  AArch64LegalizerInfo.cpp
   AArch64LoadStoreOptimizer.cpp
   AArch64MachineFunctionInfo.cpp
   AArch64MacroFusion.cpp
   AArch64MCInstLower.cpp
-  AArch64PreLegalizerCombiner.cpp
-  AArch64PostLegalizerCombiner.cpp
   AArch64PromoteConstant.cpp
   AArch64PBQPRegAlloc.cpp
   AArch64RegisterBankInfo.cpp
   AArch64RegisterInfo.cpp
   AArch64SelectionDAGInfo.cpp
   AArch64SpeculationHardening.cpp
   AArch64StackTagging.cpp
   AArch64StackTaggingPreRA.cpp