[llvm] [AMDGPU] Enhance s_waitcnt insertion before barrier for gfx12 (PR #90595)

[Jay Foad via llvm-commits]

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@@ -1386,6 +1386,14 @@ class SIInstrInfo final : public AMDGPUGenInstrInfo {
   // This is used if an operand is a 32 bit register but needs to be aligned
   // regardless.
   void enforceOperandRCAlignment(MachineInstr &MI, unsigned OpName) const;
+
+  bool isBarrierStart(uint16_t Opcode) const {
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jayfoad wrote:

Nits: should really have a function comment. Also there are load of other `is` predicates in this file, finishing around line 920. Maybe move this up there with them?

https://github.com/llvm/llvm-project/pull/90595