[LoopFlatten] Run it earlier, just before IndVarSimplify

This is a prep step for widening induction variables in LoopFlatten if this is
posssible (D90640), to avoid having to perform certain overflow checks. Since
IndVarSimplify may already widen induction variables, we want to run
LoopFlatten just before IndVarSimplify. This is a minor reshuffle as both
passes were already close after each other.

Differential Revision: https://reviews.llvm.org/D90402

Author: Sjoerd Meijer

llvm/lib/Passes/PassBuilder.cpp

@@ -535,15 +535,16 @@ PassBuilder::buildO1FunctionSimplificationPipeline(OptimizationLevel Level,
   // TODO: Investigate promotion cap for O1.
   LPM1.addPass(LICMPass(PTO.LicmMssaOptCap, PTO.LicmMssaNoAccForPromotionCap));
   LPM1.addPass(SimpleLoopUnswitchPass());
+
+  if (EnableLoopFlatten)
+    FPM.addPass(LoopFlattenPass());
   LPM2.addPass(IndVarSimplifyPass());
   LPM2.addPass(LoopIdiomRecognizePass());
 
   for (auto &C : LateLoopOptimizationsEPCallbacks)
     C(LPM2, Level);
 
   LPM2.addPass(LoopDeletionPass());
-  if (EnableLoopFlatten)
-    FPM.addPass(LoopFlattenPass());
   // Do not enable unrolling in PreLinkThinLTO phase during sample PGO
   // because it changes IR to makes profile annotation in back compile
   // inaccurate. The normal unroller doesn't pay attention to forced full unroll

llvm/lib/Transforms/IPO/PassManagerBuilder.cpp

@@ -441,17 +441,17 @@ void PassManagerBuilder::addFunctionSimplificationPasses(
   MPM.add(createCFGSimplificationPass());
   MPM.add(createInstructionCombiningPass());
   // We resume loop passes creating a second loop pipeline here.
+  if (EnableLoopFlatten) {
+    MPM.add(createLoopFlattenPass()); // Flatten loops
+    MPM.add(createLoopSimplifyCFGPass());
+  }
   MPM.add(createIndVarSimplifyPass());        // Canonicalize indvars
   MPM.add(createLoopIdiomPass());             // Recognize idioms like memset.
   addExtensionsToPM(EP_LateLoopOptimizations, MPM);
   MPM.add(createLoopDeletionPass());          // Delete dead loops
 
   if (EnableLoopInterchange)
     MPM.add(createLoopInterchangePass()); // Interchange loops
-  if (EnableLoopFlatten) {
-    MPM.add(createLoopFlattenPass()); // Flatten loops
-    MPM.add(createLoopSimplifyCFGPass());
-  }
 
   // Unroll small loops
   MPM.add(createSimpleLoopUnrollPass(OptLevel, DisableUnrollLoops,
@@ -1045,12 +1045,12 @@ void PassManagerBuilder::addLTOOptimizationPasses(legacy::PassManagerBase &PM) {
   PM.add(createMergedLoadStoreMotionPass()); // Merge ld/st in diamonds.
 
   // More loops are countable; try to optimize them.
+  if (EnableLoopFlatten)
+    PM.add(createLoopFlattenPass());
   PM.add(createIndVarSimplifyPass());
   PM.add(createLoopDeletionPass());
   if (EnableLoopInterchange)
     PM.add(createLoopInterchangePass());
-  if (EnableLoopFlatten)
-    PM.add(createLoopFlattenPass());
 
   // Unroll small loops
   PM.add(createSimpleLoopUnrollPass(OptLevel, DisableUnrollLoops,