[X86] Take advantage of the lzcnt instruction on btver2 architectures when ORing comparisons to zero.
This change adds transformations such as:
zext(or(setcc(eq, (cmp x, 0)), setcc(eq, (cmp y, 0)))) To: srl(or(ctlz(x), ctlz(y)), log2(bitsize(x))
This optimisation is beneficial on Jaguar architecture only, where lzcnt has a good reciprocal throughput.
Other architectures such as Intel's Haswell/Broadwell or AMD's Bulldozer/PileDriver do not benefit from it.
For this reason the change also adds a "HasFastLZCNT" feature which gets enabled for Jaguar.
Differential Revision: https://reviews.llvm.org/D23446