[InstCombine] squash is-power-of-2 that uses ctpop

This is another intermediate IR step towards solving PR42314:
https://bugs.llvm.org/show_bug.cgi?id=42314

We can test if a value is power-of-2-or-0 using ctpop(X) < 2,
so combining that with a non-zero check of the input is the
same as testing if exactly 1 bit is set:

(X != 0) && (ctpop(X) u< 2) --> ctpop(X) == 1

Differential Revision: https://reviews.llvm.org/D63660

llvm-svn: 364153