[llvm] [AMDGPU] Select v_lshl_add_u32 instead of v_mul_lo_u32 by constant (PR #71035)

[Matt Arsenault via llvm-commits]

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@@ -4080,6 +4080,17 @@ SDValue AMDGPUTargetLowering::performMulCombine(SDNode *N,
   SDValue N0 = N->getOperand(0);
   SDValue N1 = N->getOperand(1);
 
+  // FIXME: Probably should only be done for gfx9 and onward.
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arsenm wrote:

Add a DAG comment and explain why it's worth doing this 

https://github.com/llvm/llvm-project/pull/71035