[llvm] [RISCV] Combine vslide{up,down} x, poison -> x (PR #169013)

[Luke Lau via llvm-commits]

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@@ -21834,6 +21834,11 @@ SDValue RISCVTargetLowering::PerformDAGCombine(SDNode *N,
       return N->getOperand(0);
     break;
   }
+  case RISCVISD::VSLIDEDOWN_VL:
+  case RISCVISD::VSLIDEUP_VL:
+    if (N->getOperand(1)->isUndef())
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lukel97 wrote:

If we add an undef test then the undef detector linter will complain. 

For the use case in the loop vectorizer just handling poison is fine if we want to avoid isUndef. Are we moving away from it in SelectionDAG?

https://github.com/llvm/llvm-project/pull/169013