Cherry-pick AArch64/GlobalISel fp copy fix

[nikic]

04fb9b7 fixes a miscompile in AArch64 debug builds using GlobalISel, where previously nonsensical mrs NZCV instructions were inserted (originally reported at rust-lang/rust#92786).

The patch does not apply cleanly to 13.x, I have rebased it here: nikic@1715dad Notably the test output in the truncating_f8 test is slightly different, there are three COPYs on 13.x rather than two. I'm not sure whether that is okay or indicative of a problem (maybe @aemerson can confirm).

If it's too late for the 13.0.1 release, we'll carry this patch on the rustc side.

[llvmbot]

@llvm/issue-subscribers-backend-AArch64

[tstellar]

@TNorthover What do you think about this?

[TNorthover]

Yep, I think this one is good too.

[nikic]

Removing this as a backport candidate -- this failed rustc aarch64 testing with:

B0 = COPY H0
unimplemented reg-to-reg copy

So probably this patch has some other dependency on the main branch, but I haven't figure out what it is yet.