[InstCombine] [NFC] Add more tests for getting rid of select of bittest (D45108, PR36950 / PR17564)

Summary:
More tests for D45108:
* One use tests
* allow shift to be a variable, too

Reviewers: spatel, craig.topper

Reviewed By: spatel

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D45329

llvm-svn: 329348