[AVX-512] Support FCOPYSIGN for v16f32 and v8f64

Summary:
This extends FCOPYSIGN support to 512-bit vectors.

I've also added tests to show what the 128-bit and 256-bit cases look like with broadcast loads.

Reviewers: delena, zvi, RKSimon, spatel

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D26791

llvm-svn: 287298