[llvm] [AArch64] Stop reserved registers from being saved in prolog/epilog (PR #138448)

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

For why the machine verifier is complaining: there's a rule that every register has to be defined before it is used.  Internally, "X1" and "W1" are different registers, so defining "W1" doesn't define "X1".  (This is how it works for all registers with subregisters; it's not specific to what kind of register is involved.)

The usual way to represent this is adding an implicit-def operand to the instruction; this reflects the general aarch64 rule that defining "W1" also defines "X1".

https://github.com/llvm/llvm-project/pull/138448