[IR] Implement DataLayout::getPointerTypeSizeInBits using getPointerSizeInBits directly

[craig.topper]

Summary

Currently we use getTypeSizeInBits which contains a switch statement to dispatch based on what the Type is. We know we always have a pointer type here, but the compiler isn't able to figure out that out to remove the switch.

This patch changes it to just call handle the pointer type directly by calling getPointerSizeInBits without going through a switch.

getPointerTypeSizeInBits is called pretty often, particularly by getOrEnforceKnownAlignment which is used by InstCombine. This should speed that up a little bit.

[craig.topper]

Ping

[arsenm]

LGTM

llvm/trunk/lib/IR/DataLayout.cpp

if (I == Pointers.end() || I->AddressSpace != AS) {
     assert(I->AddressSpace == 0);
   }
   return I->TypeByteWidth;
 }
 
 unsigned DataLayout::getPointerTypeSizeInBits(Type *Ty) const {
   assert(Ty->isPtrOrPtrVectorTy() &&
          "This should only be called with a pointer or pointer vector type");
-
-  if (Ty->isPointerTy())
-    return getTypeSizeInBits(Ty);
-
-  return getTypeSizeInBits(Ty->getScalarType());
+  Ty = Ty->getScalarType();
+  return getPointerSizeInBits(cast<PointerType>(Ty)->getAddressSpace());
 }
 
 /*!
   \param abi_or_pref Flag that determines which alignment is returned. true
   returns the ABI alignment, false returns the preferred alignment.
   \param Ty The underlying type for which alignment is determined.
 
   Get the ABI (\a abi_or_pref == true) or preferred alignment (\a abi_or_pref