[NFC] Clarify comment about LoopDeletionPass in the optimization pipeline

[lebedev.ri]

FTR i will be rather surprised if that alone would fully alleviate the need for this late loopdelete.

[aeubanks]

In D115179#3174484, @lebedev.ri wrote:

FTR i will be rather surprised if that alone would fully alleviate the need for this late loopdelete.

Changing the EarlyCSEPass in PassBuilder::buildFunctionSimplificationPipeline() to GVNPass does make llvm/test/Transforms/PhaseOrdering/deletion-of-loops-that-became-side-effect-free.ll pass if we delete the -O1 RUN line

[lebedev.ri]

In D115179#3176871, @aeubanks wrote:

In D115179#3174484, @lebedev.ri wrote:

FTR i will be rather surprised if that alone would fully alleviate the need for this late loopdelete.

Changing the EarlyCSEPass in PassBuilder::buildFunctionSimplificationPipeline() to GVNPass does make llvm/test/Transforms/PhaseOrdering/deletion-of-loops-that-became-side-effect-free.ll pass if we delete the -O1 RUN line

I'm sure it does, but i think everyone understands that the test in question does not provide exhaustive test coverage for all possible affected code patterns.

llvm/lib/Passes/PassBuilderPipelines.cpp

PassBuilder::buildModuleOptimizationPipeline(OptimizationLevel Level,
   for (auto &C : VectorizerStartEPCallbacks)
     C(OptimizePM, Level);
 
   LoopPassManager LPM;
   // First rotate loops that may have been un-rotated by prior passes.
   // Disable header duplication at -Oz.
   LPM.addPass(LoopRotatePass(Level != OptimizationLevel::Oz, LTOPreLink));
   // Some loops may have become dead by now. Try to delete them.
-  // FIXME: see disscussion in https://reviews.llvm.org/D112851
-  //        this may need to be revisited once GVN is more powerful.
+  // FIXME: see discussion in https://reviews.llvm.org/D112851,
+  //        this may need to be revisited once we run GVN before loop deletion
+  //        in the simplification pipeline.
   LPM.addPass(LoopDeletionPass());
   OptimizePM.addPass(createFunctionToLoopPassAdaptor(
       std::move(LPM), /*UseMemorySSA=*/false, /*UseBlockFrequencyInfo=*/false));
 
   // Distribute loops to allow partial vectorization.  I.e. isolate dependences
   // into separate loop that would otherwise inhibit vectorization.  This is
   // currently only performed for loops marked with the metadata
   // llvm.loop.distribute=true or when -enable-loop-distribute is specified.