[GISel] Add identity and fneg combines for G_FMUL

[mkitzan]

Summary

Patch adds two simple combines:

• G_FMUL(x, 1.0) -> x
• G_FMUL(G_FNEG(x), G_FNEG(y)) -> G_FMUL(x, y)

Patch additionally adds new combine tests for AArch64 target for these new rules.
(Split off from https://reviews.llvm.org/D87870)

[foad]

General question about all these combines: why do we need them? Why wouldn't they be done at the IR level, or are they cleaning up code that could reasonably be introduced by legalization?

[mkitzan]

These are combines which seem generally beneficial (not architecture specific) from the closed source backend I work on. They are beneficial for our backend, so we have been upstreaming these combines to improve GISel.

[arsenm]

llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp

2434–2435

This is illegal, no reason to check for it. Really we should just consistently use Register in the API