[AArch64][SME] Merge back-to-back SME call regions #142111
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This patch updates the definition of `AArch64ISD::INIT_TPIDR2OBJ` to take the number of save slices (which is currently always all ZA slices). Using this, we can initialize the TPIDR2 block with a single STP of the save buffer pointer and the number of save slices. The reserved bytes (10-15) will be implicitly zeroed as the result of RDSVL will always be <= 16-bits. Using an STP is also possible for big-endian targets with an additional left shift. Note: We used to write the number of save slices to the TPIDR2 block before every call with a lazy save; however, based on 6.6.9 "Changes to the TPIDR2 block" in the aapcs64 (https://github.com/ARM-software/abi-aa/blob/main/aapcs64/aapcs64.rst#changes-to-the-tpidr2-block), it seems we can rely on callers preserving the contents of the TPIDR2 block.
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Proof-of-concept patch for merging back-to-back SME call regions (e.g. lazy saves, zt0 spills, streaming mode switches).