[AArch64][SME][SDAG] Add basic support for exception handling (#159363)

This patch adds basic support for exception handling to SelectionDAG for
ZT0, ZA, and agnostic ZA state. This works based on the following
assumptions:

- To throw an exception requires calling into the runtime
  * The which will be a private ZA call (that commits the lazy save)
- Therefore, as noted in https://github.com/ARM-software/abi-aa/blob/main/aapcs64/aapcs64.rst#exceptions
we will always enter the EH block with PSTATE.ZA=0 and TPIDR2_EL0=null,
so we can emit a restore of ZA/ZT0.

Note: This patch does not handle all cases yet. Currently, there is no
support for committing agnostic ZA state before `invoke`s, regardless of
whether the callee is also agnostic (to ensure ZA state is saved on all
normal returns).

0 files changed, 0 insertions, 0 deletions