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

[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@354299 91177308-0d34-0410-b5e6-96231b3b80d8