[x86, SSE/AVX] allow 128/256-bit lowering for copysign vector intrinsics (PR30433)

This should fix:
https://llvm.org/bugs/show_bug.cgi?id=30433

There are a couple of open questions about the codegen:
1. Should we let scalar ops be scalars and avoid vector constant loads/splats?
2. Should we have a pass to combine constants such as the inverted pair that we have here?

Differential Revision: https://reviews.llvm.org/D25165
 

llvm-svn: 283119