x64: Add support for fmsub and fnmsub instructions

[afonso360]

👋 Hey,

Feature

Add support for the fmsub and fnmsub instructions in the x64 backend.

Benefit

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

Implementation

We already have support for the fmadd and fnmadd versions of these instructions here and 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!

[UnlimitedHummus]

Hey, I would like to try to implement this. I might need some help, as this is my first contribution.

If I have questions, should I ask them here or in Zulip?

[afonso360]

Awesome! 🎉

If I have questions, should I ask them here or in Zulip?

Up to you, I'm available in both. I might be able to give you a faster reply on Zulip since that usually sends me a notification.