This patch adds a new `TargetLowering` hook `lowerEHPadEntry()` that is
called at the start of lowering EH pads in SelectionDAG. This allows the
insertion of target-specific actions on entry to exception handlers.
This is used on AArch64 to insert SME streaming-mode switches at landing
pads. This is needed as exception handlers are always entered with
PSTATE.SM off, and the function needs to resume the streaming mode of
the function body.
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