[llvm] [Thumb1] Resolve FIXME: use 'mov hi, $src; mov $dst, hi' (PR #81908)

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

For r10 and r11, as the tests show, they're almost never available; the compiler won't naturally spill them.  You can force them to spill using inline asm, but that's so rare it's not really relevant.  So I don't really see the point of checking, unless you're planning some sort of followup.

For r12... this is probably okay?  We don't use r12 in the scavenger anymore (see ab1d73ee).  We still use R12 as a scratch register in a few different places, but not in a way that would conflict, I think.  It would be helpful if you could take some time to verify that.

If there's some way r12 can actually be live (inline asm?), we should make sure there's at least one test that actually still exercises the code to generate push/pop.

https://github.com/llvm/llvm-project/pull/81908