[CodeGen] Use pointer-sized integers for ptrtoint sources

Given something like:
void *v = (void *)100;

We need to synthesize a ptrtoint operation from 100.  During constant
emission, we choose i64 as the type for our constant because it
guaranteed not to drop any bits from our CharUnits representation of the
value.  However, this is suboptimal for 32-bit targets: LLVM passes like
GlobalOpt will get confused by these sorts of casts resulting in
pessimization.

Instead, make sure the ptrtoint operand has a pointer-sized integer
type.

llvm-svn: 273020