[WIP] [SeparateConstOffsetFromGEP] Use DisableSeparateConstOffsetFromGEP option with legacy PM

[djtodoro]

Summary

This just improves a bit the SeparateConstOffsetFromGEP Pass.

llvm/lib/Transforms/Scalar/SeparateConstOffsetFromGEP.cpp

bool SeparateConstOffsetFromGEP::splitGEP(GetElementPtrInst *GEP) {
   GEP->eraseFromParent();
 
   return true;
 }
 
 bool SeparateConstOffsetFromGEPLegacyPass::runOnFunction(Function &F) {
   if (skipFunction(F))
     return false;
+
+  if (DisableSeparateConstOffsetFromGEP)
+    return false;
+
   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);