[GlobalISel] Remove scalar src from non-sequential fadd/fmul reductions.

It's probably better to split these into separate G_FADD/G_FMUL + G_VECREDUCE
operations in the translator rather than carrying the scalar around. The
majority of the time it'll get simplified away as the scalars are probably
identity values.

Differential Revision: https://reviews.llvm.org/D89150