[x86] invert a vector select IR canonicalization with a binop identity constant

This is an intentionally limited/different form of D90113.
That patch bravely tries to generalize folds where we pull
a binop into the arms of a select:
N0 + (Cond ? 0 : FVal) --> Cond ? N0 : (N0 + FVal)
...but it is not universally profitable.

This is the inverse of IR canonicalization as discussed in
D113442.

We know that this transform is not entirely profitable even
within x86, so we only handle x86 vector fadd/fsub as a 1st
step. The intent is to prevent AVX512 regressions as mentioned
in D113442.

The plan is to port this to DAGCombiner (so it will eventually
look more like D90113) and add more types/cases in pieces with
many more tests to verify that we are seeing improvements.

Differential Revision: https://reviews.llvm.org/D118644