[InstSimplify] add tests for known-not-a-power-of-2; NFC

I added a canonicalization to create this general pattern in:
rL363956

But as noted in PR42314:
https://bugs.llvm.org/show_bug.cgi?id=42314#c11

...we have a (potentially expensive) simplification for the version
of the code that we just canonicalized away from, so we should
add/adjust that code to match.

llvm-svn: 363981