[InstCombine] Unfold masked merge with constant mask

Summary:
As discussed in D45733, we want to do this in InstCombine.

https://rise4fun.com/Alive/LGk

Reviewers: spatel, craig.topper

Reviewed By: spatel

Subscribers: chandlerc, xbolva00, llvm-commits

Differential Revision: https://reviews.llvm.org/D45867

llvm-svn: 331205