[AArch64] Add Machine InstCombiner patterns for FMUL indexed variant

This patch adds DUP+FMUL => FMUL_indexed pattern to InstCombiner.
FMUL_indexed is normally selected during instruction selection, but it
does not work in cases when VDUP and VMUL are in different basic
blocks.

Differential Revision: https://reviews.llvm.org/D99662