[AArch64] Fix MUL/SUB fusing

Summary:
When MUL is the first operand to SUB, we can't use MLS because the accumulator
should be negated.  Emit a NEG of the accumulator and an MLA instead, similar to
what we do for FMUL / FSUB fusing.

Reviewers: dmgreen, SjoerdMeijer, fhahn, Gerolf, mstorsjo, asbirlea

Reviewed By: asbirlea

Subscribers: kristof.beyls, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71067