[X86][SSE] Change CTTZ vector lowering to cttz(x) = ctpop(~x & (x - 1))

This patch changes the vector CTTZ lowering from:

cttz(x) = ctpop((x & -x) - 1)

to:

cttz(x) = ctpop(~x & (x - 1))

Not only does this make better use of the PANDN instruction, but it also matches the LegalizeDAG method which should allow us to remove the x86 specific code at some point in the future (we need to fix some issues with the bitcasted logic ops and CTPOP lowering first).

Differential Revision: https://reviews.llvm.org/D53214

llvm-svn: 344447