[InstCombine] [NFC] Add tests for getting 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, D45108

Reviewers: spatel, craig.topper

Reviewed By: spatel

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D45107

llvm-svn: 329198