[InstCombine] Teach how to fold a select into a cttz/ctlz with the 'is_zero_undef' flag.

This patch teaches the Instruction Combiner how to fold a cttz/ctlz followed by
a icmp plus select into a single cttz/ctlz with flag 'is_zero_undef' cleared.

Added test InstCombine/select-cmp-cttz-ctlz.ll.

llvm-svn: 227197