[flang][NFC] Rename RewriteLoop.cpp to ControlFlowConverter.cpp

[clementval]

Summary

RewriteLoop.cpp is containing more than just loop conversion. It will
soon contains the fir.select_type conversion as well. This patch
renames the file so it is in line with the pass name.

[jeanPerier]

LGTM

flang/lib/Optimizer/Transforms/CMakeLists.txt

 add_flang_library(FIRTransforms
   AbstractResult.cpp
   AffinePromotion.cpp
   AffineDemotion.cpp
   AnnotateConstant.cpp
   CharacterConversion.cpp
+  ControlFlowConverter.cpp
   ArrayValueCopy.cpp
   ExternalNameConversion.cpp
   MemoryAllocation.cpp
   MemRefDataFlowOpt.cpp
-  RewriteLoop.cpp
   SimplifyRegionLite.cpp
   AlgebraicSimplification.cpp
   SimplifyIntrinsics.cpp
 
   DEPENDS
   FIRBuilder
   FIRDialect
   FIRSupport

flang/lib/Optimizer/Transforms/ControlFlowConverter.cpp

-//===-- RewriteLoop.cpp ---------------------------------------------------===//
+//===-- ControlFlowConverter.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
 //
 //===----------------------------------------------------------------------===//
 
 #include "flang/Optimizer/Dialect/FIRDialect.h"