[APInt] Introduce APIntOps::GetMostSignificantDifferentBit()

Summary:
Compare two values, and if they are different, return the position of the
most significant bit that is different in the values.

Needed for D69387.

Reviewers: nikic, spatel, sanjoy, RKSimon

Reviewed By: nikic

Subscribers: xbolva00, hiraditya, dexonsmith, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D69439