[LoopReroll] Remove unused and unmaintained pass (#80972)

Remove the LoopReroll pass, which is both unused (in any default
pipeline) and unmaintained, with numerous open correctness issues
(https://github.com/llvm/llvm-project/issues?q=is%3Aissue+is%3Aopen+loop-reroll).
The removal is in line with
https://discourse.llvm.org/t/rfc-disallow-unmaintained-unused-passes/75151.

There is also a defunct `-freroll-loops` option in clang, which I'll
remove separately.

Migrated from https://reviews.llvm.org/D150684.

Author: nikic

llvm/include/llvm/Transforms/Scalar/LoopReroll.h

@@ -216,7 +216,6 @@
 #include "llvm/Transforms/Scalar/LoopLoadElimination.h"
 #include "llvm/Transforms/Scalar/LoopPassManager.h"
 #include "llvm/Transforms/Scalar/LoopPredication.h"
-#include "llvm/Transforms/Scalar/LoopReroll.h"
 #include "llvm/Transforms/Scalar/LoopRotation.h"
 #include "llvm/Transforms/Scalar/LoopSimplifyCFG.h"
 #include "llvm/Transforms/Scalar/LoopSink.h"

llvm/lib/Passes/PassBuilder.cpp

@@ -599,7 +599,6 @@ LOOP_PASS("loop-idiom", LoopIdiomRecognizePass())
 LOOP_PASS("loop-instsimplify", LoopInstSimplifyPass())
 LOOP_PASS("loop-predication", LoopPredicationPass())
 LOOP_PASS("loop-reduce", LoopStrengthReducePass())
-LOOP_PASS("loop-reroll", LoopRerollPass())
 LOOP_PASS("loop-simplifycfg", LoopSimplifyCFGPass())
 LOOP_PASS("loop-unroll-full", LoopFullUnrollPass())
 LOOP_PASS("loop-versioning-licm", LoopVersioningLICMPass())

llvm/lib/Passes/PassRegistry.def

@@ -40,7 +40,6 @@ add_llvm_component_library(LLVMScalarOpts
   LoopLoadElimination.cpp
   LoopPassManager.cpp
   LoopPredication.cpp
-  LoopRerollPass.cpp
   LoopRotation.cpp
   LoopSimplifyCFG.cpp
   LoopStrengthReduce.cpp