[llvm] [AMDGPU] Update hasUnwantedEffectsWhenEXECEmpty (PR #97982)

[Matt Arsenault via llvm-commits]

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@@ -4118,6 +4118,13 @@ bool SIInstrInfo::modifiesModeRegister(const MachineInstr &MI) {
 }
 
 bool SIInstrInfo::hasUnwantedEffectsWhenEXECEmpty(const MachineInstr &MI) const {
+  // This function is used to determine if an instruction can be safely
+  // executed under EXECZ without hardware error, indeterminate results,
+  // and/or visible effects on future vector execution or outside the shader.
+  // Note: as of 2024 the only use of this is SIPreEmitPeephole where it is
+  // used in removing branches over short EXECZ sequences.
+  // As such it embeds certain assumptions which may not apply in every case
+  // of EXECZ execution.
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arsenm wrote:

You duplicated it, only keep the header one 

https://github.com/llvm/llvm-project/pull/97982