InstCombine: Optimize GEP's involving ptrtoint better

We supported transforming:
(gep i8* X, -(ptrtoint Y))

to:
(inttoptr (sub (ptrtoint X), (ptrtoint Y)))

However, this only fired if 'X' had type i8*.  Generalize this to
support various types of different sizes.  This results in much better
CodeGen, especially for pointers to packed structs.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216523 91177308-0d34-0410-b5e6-96231b3b80d8