[ConstantHoisting] Cache OptForSize. (#79170)

CacheOptForSize to remove quadratic behavior.

For each constant analyzed, ConstantHoising calls
`shouldOptimizeForSize(F)`, which calls `PSI.getTotalCallCount(F)`.
PSI.getTotalCallCount(F) goes through all the instructions in all basic
blocks, and checks if each is a call, to count them up.

This reduces `llc` time for a very large IR from ~10min to under 3min.
Reproducer testcase is much too large to share.

Author: alinas

llvm/include/llvm/Transforms/Scalar/ConstantHoisting.h

@@ -153,6 +153,7 @@ class ConstantHoistingPass : public PassInfoMixin<ConstantHoistingPass> {
   const DataLayout *DL;
   BasicBlock *Entry;
   ProfileSummaryInfo *PSI;
+  bool OptForSize;
 
   /// Keeps track of constant candidates found in the function.
   using ConstCandVecType = std::vector<consthoist::ConstantCandidate>;

llvm/lib/Transforms/Scalar/ConstantHoisting.cpp

@@ -576,9 +576,6 @@ ConstantHoistingPass::maximizeConstantsInRange(ConstCandVecType::iterator S,
                                            ConstCandVecType::iterator &MaxCostItr) {
   unsigned NumUses = 0;
 
-  bool OptForSize = Entry->getParent()->hasOptSize() ||
-                    llvm::shouldOptimizeForSize(Entry->getParent(), PSI, BFI,
-                                                PGSOQueryType::IRPass);
   if (!OptForSize || std::distance(S,E) > 100) {
     for (auto ConstCand = S; ConstCand != E; ++ConstCand) {
       NumUses += ConstCand->Uses.size();
@@ -948,6 +945,10 @@ bool ConstantHoistingPass::runImpl(Function &Fn, TargetTransformInfo &TTI,
   this->Ctx = &Fn.getContext();
   this->Entry = &Entry;
   this->PSI = PSI;
+  this->OptForSize = Entry.getParent()->hasOptSize() ||
+                     llvm::shouldOptimizeForSize(Entry.getParent(), PSI, BFI,
+                                                 PGSOQueryType::IRPass);
+
   // Collect all constant candidates.
   collectConstantCandidates(Fn);