[x86] convert logic-of-FP-compares to FP logic-of-vector-compares

This is motivated by the examples and discussion in:
https://llvm.org/PR51245
...and related bugs.

By using vector compares and vector logic, we can convert 2 'set'
instructions into 1 'movd' or 'movmsk' and generally improve
throughput/reduce instructions.

Unfortunately, we don't have a complete vector compare ISA before
AVX, so I left SSE-only out of this patch. Ie, we'd need extra logic
ops to simulate the missing predicates for SSE 'cmpp*', so it's not
as clearly a win.

Differential Revision: https://reviews.llvm.org/D110342