[AArch64] Combine vector fptoi.sat(fmul) to fixed point fcvtz

Similar to D113199 but dealing with the vector size, this extends the
fptosi+fmul to fixed point fold to handle fptosi.sat nodes that are
equally viable, so long as the saturation width matches the output
width.

Differential Revision: https://reviews.llvm.org/D113200