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

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

Pretty sure that's just wrong (or referring to some other calling convention).  From the specification (https://github.com/ARM-software/abi-aa/releases):

> A subroutine must preserve the contents of the registers r4-r8, r10, r11 and SP (and r9 in PCS variants that designate r9 as v6).

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