[SME] Don't scavenge a spillslot in callee-save area in presence of streaming-mode changes.

If no frame-pointer is available and the compiler has scavenged a
spill-slot in the callee-save area, the compiler may be forced to emit an
'addvl' inside the streaming-mode-changing call sequence when it needs to
fill (reload) an FP register being passed to the call.

We can avoid this entirely by disabling stack-slot scavenging when there
are streaming-mode-changing call-sequences in the function.

Reviewed By: david-arm

Differential Revision: https://reviews.llvm.org/D159196