[llvm] [AMDGPU][SIInsertWaitcnts] drop OldWaitcntInstr only when it is processed (PR #145720)

[Juan Manuel Martinez Caamaño via llvm-commits]

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@@ -2490,7 +2493,6 @@ bool SIInsertWaitcnts::insertWaitcntInBlock(MachineFunction &MF,
     // Generate an s_waitcnt instruction to be placed before Inst, if needed.
     Modified |= generateWaitcntInstBefore(Inst, ScoreBrackets, OldWaitcntInstr,
                                           FlushVmCnt);
-    OldWaitcntInstr = nullptr;
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jmmartinez wrote:

If I follow everything, the main case where assigning `OldWaitcntInstr` to `nullptr` here was problematic was for meta-instructions ? Or is there another case ?

If that's the case, we could skip meta-instructions directly in the this loop.

https://github.com/llvm/llvm-project/pull/145720