CodeGenPrepare: add target hook to enable address sinking using GEPs

[escha]

Summary

Split from my previous review and turned into a TLI option.

[arsenm]

This is the default now so probably not needed anymore

include/llvm/Target/TargetLowering.h

     return false;
   }
 
+  /// Return true if CodeGenPrepare should sink addresses using GetElementPtr
+  /// instructions.
+  virtual bool shouldSinkAddressUsingGEP() const {
+    return false;
+  }
+
   //===--------------------------------------------------------------------===//
   // Runtime Library hooks
   //

lib/CodeGen/CodeGenPrepare.cpp

                  << *MemoryInst << "\n");
     if (SunkAddr->getType() != Addr->getType())
       SunkAddr = Builder.CreateBitCast(SunkAddr, Addr->getType());
-  } else if (AddrSinkUsingGEPs ||
+  } else if (AddrSinkUsingGEPs || TLI->shouldSinkAddressUsingGEP() ||
              (!AddrSinkUsingGEPs.getNumOccurrences() && TM &&
               TM->getSubtargetImpl(*MemoryInst->getParent()->getParent())
                   ->useAA())) {