[x86] fix FCOPYSIGN lowering to create constants instead of ConstantPool loads

This is similar to:
https://reviews.llvm.org/rL279958

By not prematurely lowering to loads, we should be able to more easily eliminate
the 'or' with zero instructions seen in copysign-constant-magnitude.ll.

We should also be able to extend this code to handle vectors.

llvm-svn: 282312