[Loads] Fix crash in is isDereferenceableAndAlignedPointer()

Summary:
We were trying to add APInt values with different bit sizes after
visiting an addrspacecast instruction which changed the bit width
of the pointer.

Reviewers: majnemer, hfinkel

Subscribers: hfinkel, wdng, llvm-commits

Differential Revision: https://reviews.llvm.org/D24774

llvm-svn: 285407