[X86] Recognize a splat of negate in isFNEG

Summary:
Expand isFNEG so that we generate the appropriate F(N)M(ADD|SUB)
instructions in more cases. For example, the following sequence
a = _mm256_broadcast_ss(f)
d = _mm256_fnmadd_ps(a, b, c)

generates an fsub and fma without this patch and an fnma with this
change.

Reviewers: craig.topper

Subscribers: llvm-commits, davidxl, wmi

Differential Revision: https://reviews.llvm.org/D48467

llvm-svn: 339043