[WIP] [SeparateConstOffsetFromGEP] Add an option to enable GEP lowering

[djtodoro]

Summary

This could be useful for testing of the SeparateConstOffsetFromGEP Pass.

llvm/lib/Transforms/Scalar/SeparateConstOffsetFromGEP.cpp

 // Setting this flag may emit false positives when the input module already
 // contains dead instructions. Therefore, we set it only in unit tests that are
 // free of dead code.
 static cl::opt<bool>
     VerifyNoDeadCode("reassociate-geps-verify-no-dead-code", cl::init(false),
                      cl::desc("Verify this pass produces no dead code"),
                      cl::Hidden);
 
+static cl::opt<bool> LowerGEPOpt("lower-gep", cl::init(false),
+                                 cl::desc("Enable GEP lowering"), cl::Hidden);
+
 namespace {
 
 /// A helper class for separating a constant offset from a GEP index.
 ///
 /// In real programs, a GEP index may be more complicated than a simple addition
 /// of something and a constant integer which can be trivially splitted. For
 /// example, to split ((a << 3) | 5) + b, we need to search deeper for the
 /// constant offset, so that we can separate the index to (a << 3) + b and 5.

 
 bool SeparateConstOffsetFromGEPLegacyPass::runOnFunction(Function &F) {
   if (skipFunction(F))
     return false;
 
   if (DisableSeparateConstOffsetFromGEP)
     return false;
 
+  if (LowerGEPOpt)
+    LowerGEP = true;
+
   auto *DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
   auto *SE = &getAnalysis<ScalarEvolutionWrapperPass>().getSE();
   auto *LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
   auto *TLI = &getAnalysis<TargetLibraryInfoWrapperPass>().getTLI(F);
   auto GetTTI = [this](Function &F) -> TargetTransformInfo & {
     return this->getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
   };
   SeparateConstOffsetFromGEP Impl(DT, SE, LI, TLI, GetTTI, LowerGEP);
   return Impl.run(F);
 }
 
 bool SeparateConstOffsetFromGEP::run(Function &F) {
   if (DisableSeparateConstOffsetFromGEP)
     return false;
 
+  if (LowerGEPOpt)
+    LowerGEP = true;
+
   DL = &F.getParent()->getDataLayout();
   bool Changed = false;
   for (BasicBlock &B : F) {
     for (BasicBlock::iterator I = B.begin(), IE = B.end(); I != IE;)
       if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(I++))
         Changed |= splitGEP(GEP);
     // No need to split GEP ConstantExprs because all its indices are constant
     // already.