[llvm] [LLVM][AArch64][CodeGen] Mark FFR as a reserved register. (PR #83437)

[David Green via llvm-commits]

davemgreen wrote:

> One thing I wondered was why the scheduling has slightly changed. Is one of the regexes perhaps missing a `$` at the end causing it to match more than it should?

Could it be that the old psuedos were not getting the scheduling info they should have had? (Removing side-effects might have helped too).

Removing excessive pseudos is often a great thing to help in general, but specifically can help simplify scheduling models.

https://github.com/llvm/llvm-project/pull/83437