[AArch64] Look through copy in MachineCombiner FMUL patterns.

This is a small addition to D99662, which added machine combiner
patterns for FMUL(DUP(..)). Due to the way these are generated from
ISel, they may also be FMUL(COPY(DUP(..))), which this patch now
ignores the no-op COPY in.

Differential Revision: https://reviews.llvm.org/D126632