[InstCombine] Get rid of select of bittest (PR36950 / PR17564)

Summary:
See [[ https://bugs.llvm.org/show_bug.cgi?id=36950 | PR36950 ]], [[ https://bugs.llvm.org/show_bug.cgi?id=17564 | PR17564 ]], D45065, D45107
https://godbolt.org/g/iAYRup

Alive proof: https://rise4fun.com/Alive/uiH

Testing: `ninja check-llvm`

Reviewers: spatel, craig.topper

Reviewed By: spatel

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D45108

llvm-svn: 329492