aarch64: Add support for fnmadd/fmsub/fnmsub

[afonso360]

👋 Hey,

Feature

Add support for the fnmadd / fmsub / fnmsub instructions in the aarch64 backend.

Benefit

This improves instruction selection quality for a few special fma cases.

Implementation

We already have support for the scalar fmadd instruction here. So we adding support for these instructions should be a matter of introducing the new opcodes and implementing lowering rules similar to what already exists today.

Something similar was done in #8588 for the RISC-V backend. If anyone needs help implementing this, let me know!