This constrains the Mov* and similar pseudo instruction to take
GPR64common register classes rather than GPR64. GPR64 includs XZR
which is invalid here, because this pseudo instructions expands
into an adrp/add pair sharing a destination register. XZR is invalid
on add and attempting to encode it will instead increment the stack
pointer causing crashes (downstream report at ). The test case
there reproduces on LLVM11, but I do not have a test case that
reaches this code path on main, since it is being masked by
improved dead code elimination introduced in D91513. Nevertheless,
this seems like a good thing to fix in case there are other cases
that dead code elimination doesn't clean up (e.g. if optnone is
used and the optimization is skipped).
I think it would be worth auditing uses of GPR64 in pseudo
instructions to see if there are any similar issues, but I do not
have a high enough view of the backend or knowledge of the
Aarch64 architecture to do this quickly.