[llvm] [SelectionDAG][AMDGPU] Preserve known bits for demoted sret pointers (PR #203468)

[Matt Arsenault via llvm-commits]

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@@ -4427,6 +4427,12 @@ class LLVM_ABI TargetLowering : public TargetLoweringBase {
                                              KnownBits &Known,
                                              const MachineFunction &MF) const;
 
+  /// Determine which bits of a stack object pointer are known to be 0.
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arsenm wrote:

```suggestion
  /// Determine known bits of a pointer to a known valid stack object.
```

It could also tell if bits are 1

https://github.com/llvm/llvm-project/pull/203468