[AArch64][SME] Treat agnostic ZA invokes like private ZA callees (#162684)

An invoke of an agnostic ZA function behaves like a private ZA callee.
If the invoke does not return normally (and we end up in an exception
block in the caller), ZA must be committed to the caller's save buffer
(and off).

We can ensure this by setting up a ZA save before an agnostic ZA invoke.
This will result in the agnostic ZA invoke committing ZA to its caller's
save buffer, rather than its local buffer, which allows us to reload the
correct contents of ZA within exception blocks.

Note: This also means we must restore ZA on the non-exceptional path
from the `invoke` (since ZA could have been committed to the save buffer
in either case).

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