[clang] [llvm] [ARM] Fix musttail calls (PR #109943)

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

> I think we'd get decent code by doing a sequence of LOAD <all outgoing byval bytes to vregs> ; TOKENFACTOR ; STORE <all vregs to byval argument slots>. 

Up to some threshold, sure.  If the scheduler is sufficiently clever, we won't even get any spills: we can interleave the loads and stores.  But we probably still need some fallback path that avoids emitting large numbers of inline load/store ops.

https://github.com/llvm/llvm-project/pull/109943