[VectorCombine] convert scalar fneg with insert/extract to vector fneg

insertelt DestVec, (fneg (extractelt SrcVec, Index)), Index --> shuffle DestVec, (fneg SrcVec), Mask

This is a specialized form of what could be a more general fold for a binop.
It's also possible that fneg is overlooked by SLP in this kind of
insert/extract pattern since it's a unary op.

This shows up in the motivating example from #issue 58139, but it won't solve
it (that probably requires some x86-specific backend changes). There are also
some small enhancements (see TODO comments) that can be done as follow-up
patches.

Differential Revision: https://reviews.llvm.org/D135278