[Inline] Use AssumptionCache from the right Function

This changes the behavior of AddAligntmentAssumptions to match its
comment. I.e, prove the asserted alignment in the context of the caller,
not the callee.

Thanks to Mehdi Amini for seeing the issue here! Also to Artur Pilipenko
who also saw a fix for the issue.

rdar://22521387

Differential Revision: http://reviews.llvm.org/D12997

llvm-svn: 248390