[llvm][IRBuilder] Introspection for CreateAlignmentAssumption*() functions

Summary:
Clang calls these functions to produce IR for assume-aligned attributes.
I would like to teach UBSAN to verify these assumptions.
For that, i need to access the final pointer on which the check is performed,
and the actual `icmp` that does the check.

The alternative to this would be to fully re-implement this in clang.

This is a second commit, the original one was r351104,
which was mass-reverted in r351159 because 2 compiler-rt tests were failing.

Reviewers: spatel, dneilson, craig.topper, dblaikie, hfinkel

Reviewed By: hfinkel

Subscribers: hfinkel, llvm-commits

Differential Revision: https://reviews.llvm.org/D54588

llvm-svn: 351176