[X86][SSE] Provide execution domains for scalar floating point operations

This is an updated version of Chandler's patch D7402 that got accepted but never committed, and has bit-rotted a bit since.

I've updated the execution domain declarations to match the approach of the packed templates and also added some extra scalar unary tests.

Differential Revision: http://reviews.llvm.org/D9095

llvm-svn: 235372