[GlobalISel][AArch64] Legalize + select some llvm.ctlz.* intrinsics

Legalize/select llvm.ctlz.*

Add select-ctlz to show that we actually select them. Update arm64-clrsb.ll and
arm64-vclz.ll to show that we perform valid transformations in optimized builds,
and document where GISel can improve.

Differential Revision: https://reviews.llvm.org/D58155

llvm-svn: 354299