Add a new helper method to Value to strip in-bounds constant offsets of

Add a new helper method to Value to strip in-bounds constant offsets of
pointers, but accumulate the offset into an APInt in the process of
stripping it.

This is a pretty handy thing to have, such as when trying to determine
if two pointers are at some constant relative offset. I'll be committing
a patch shortly to use it for exactly that purpose.

llvm-svn: 189000