[DAG] Emit table lookup from TargetLowering::expandCTTZ()

This patch emits table lookup in expandCTTZ.

Context -
https://reviews.llvm.org/D113291 transforms set of IR instructions to
cttz intrinsic but there are some targets which does not support CTTZ or
CTLZ. Hence, I generate a table lookup in TargetLowering::expandCTTZ().

Differential Revision: https://reviews.llvm.org/D128911