[ubsan] Skip alignment checks which are folded away

Don't emit alignment checks which the IR constant folder throws away.

I've tested this out on X86FastISel.cpp. While this doesn't decrease
end-to-end compile-time significantly, it results in 122 fewer type
checks (1% reduction) overall, without adding any real complexity.

Differential Revision: https://reviews.llvm.org/D37544

llvm-svn: 314752