[llvm] [AMDGPU] SIPeepholeSDWA: Disable on existing SDWA instructions (PR #123942)

[Matt Arsenault via llvm-commits]

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@@ -963,7 +963,7 @@ bool isConvertibleToSDWA(MachineInstr &MI,
   // Check if this is already an SDWA instruction
   unsigned Opc = MI.getOpcode();
   if (TII->isSDWA(Opc))
-    return true;
+    return false;
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arsenm wrote:

Add fixme to do this 

https://github.com/llvm/llvm-project/pull/123942