[llvm] [AMDGPU] Add dereferenceable retAttr to a call to llvm.amdgcn.implicitarg.ptr (PR #182206)

[Matt Arsenault via llvm-commits]

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@@ -722,6 +722,34 @@ std::optional<Instruction *>
 GCNTTIImpl::instCombineIntrinsic(InstCombiner &IC, IntrinsicInst &II) const {
   Intrinsic::ID IID = II.getIntrinsicID();
   switch (IID) {
+  case Intrinsic::amdgcn_implicitarg_ptr: {
+    // Not checking calling function's "amdgpu-no-implicitarg-ptr" attribute
+    // under the assumption that a call to this intrinsic is not present with
+    // that attribute.
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arsenm wrote:

In a follow up you could check the attribute and fold to poison 

https://github.com/llvm/llvm-project/pull/182206