[llvm] [AArch64][SVE] Teach compiler to use information that there are no ac… (PR #68698)

[Paul Walker via llvm-commits]

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@@ -1169,6 +1169,8 @@ instCombineSVEVectorFuseMulAddSub(InstCombiner &IC, IntrinsicInst &II,
     AddendOp = II.getOperand(2);
     Mul = II.getOperand(1);
   }
+  if (match(P, m_ZeroInt()))
+    return IC.replaceInstUsesWith(II, UndefValue::get(II.getType()));
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paulwalker-arm wrote:

Is this safe? I can see `instCombineSVEVectorFuseMulAddSub` is called by `instCombineSVEVectorFAdd` whose result for an all inactive predicate is defined.

As with calling `instCombineSVEAllActive` you might need to move this functionality into the relevant places that call this function.

https://github.com/llvm/llvm-project/pull/68698