[SelectionDAG][AMDGPU] Preserve known bits for demoted sret pointers (#203468) AMDGPU marks sret pointers with high-zero known bits so stores can be folded into MUBUF base+offset addressing. Explicit sret arguments keep this information through an AssertZext, but implicit sret lowering passes the hidden return pointer through `DemoteRegister` as a `CopyToReg/CopyFromReg` pair, where the fact is not visible to SelectionDAG known-bits queries. Add a `TargetLowering` hook for sret pointer known bits and use a shared helper to materialize those bits as an `AssertZext` for both explicit and demoted sret pointers. Validated with llvm-test-depends, the AMDGPU function-returns test, and the full llvm/test/CodeGen suite. Assisted-by: Codex --------- Signed-off-by: Keshav Vinayak Jha <keshavvinayakjha@gmail.com>
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