[CFLAA] Check for pointer types in more places.

This patch fixes an assertion that fires when we try to add non-pointer
Values to the CFLGraph. Centralizing the check for whether something
is/isn't a pointer type isn't completely trivial (and, in some cases,
would end up being entirely redundant), but it may be beneficial to do
so if this trips us up more in the future.

Patch by Jia Chen.

Differential Revision: https://reviews.llvm.org/D22947

llvm-svn: 277096