[PATCH] Resolve issues with fmuladd intrinsic handling across multiple backends

[Stephen Lin]

> Right - isFMAFasterThanMulAndAdd originally checked for legality (hence the
> comment), but that was removed to support fusing in cases where the code
> would eventually legalize.
>
> This patch looks good to me. Thanks for working on it.
>
> - Lang.

Thanks for the review! Let me know if you figure out what the ARM
backend should be doing. (that can be a separate patch, though).

Stephen