Fix a really annoying "bug" introduced in r165941. The change from that

revision makes no sense. We cannot use the address space of the *post
indexed* type to conclude anything about a *pre indexed* pointer type's
size. More importantly, this index can never be over a pointer. We are
indexing over arrays and vectors here.

Of course, I have no test case here. Neither did the original patch. =/

llvm-svn: 166091