[llvm] [ARM]Adjust cost of muls in SMLAL patterns (PR #122713)

[Nashe Mncube via llvm-commits]

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@@ -1458,16 +1458,62 @@ InstructionCost ARMTTIImpl::getArithmeticInstrCost(
   if (LooksLikeAFreeShift())
     return 0;
 
+  // When targets have both DSP and MVE we find that the
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nasherm wrote:

Sure. Seems to make sense

https://github.com/llvm/llvm-project/pull/122713