[llvm] [AMDGPU] Add llvm.amdgcn.dead intrinsic (PR #123190)

[Nicolai Hähnle via llvm-commits]

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@@ -3450,4 +3450,11 @@ def int_amdgcn_addrspacecast_nonnull : DefaultAttrsIntrinsic<
   [llvm_anyptr_ty], [llvm_anyptr_ty],
   [IntrNoMem, IntrSpeculatable]
 >;
+
+/// Make it clear to the backend that this value is really dead. For instance,
+/// when used as an input to a phi node, it will make it possible for the
+/// backend to allocate the dead lanes for operations within the corresponding
+/// incoming block.
+def int_amdgcn_dead: DefaultAttrsIntrinsic<[llvm_i32_ty], [],
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nhaehnle wrote:

We may want to overload this as `llvm_any_ty` at some point, but I suppose for now this is okay.

https://github.com/llvm/llvm-project/pull/123190