[LoopIdiomRecognize] Add CTTZ support

Summary:
Existing LIR recognizes CTLZ where shifting input variable right until it is zero. (Shift-Until-Zero idiom)

This commit:
1. Augments Shift-Until-Zero idiom to recognize CTTZ where input variable is shifted left.
2. Prepare for BitScan idiom recognition.

Patch by Yuanfang Chen (tabloid.adroit)

Reviewers: craig.topper, evstupac

Reviewed By: craig.topper

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D55876

llvm-svn: 350074