[InstCombine] Relax cttz/ctlz with select on zero

The cttz/ctlz intrinsics have a parameter specifying whether the

result is undefined for zero. cttz(x, false) can be relaxed to

cttz(x, true) if x is known non-zero, and in fact such an optimization

is already performed. However, this currently doesn't work if x is

non-zero as a result of a select rather than an explicit branch.

This patch adds handling for this case, thus allowing

x != 0 ? cttz(x, false) : y to simplify to x != 0 ? cttz(x, true) : y.

Differential Revision: https://reviews.llvm.org/D55786