[llvm] [TTI][AArch64] Add preferFixedIfEqualToScalable hook (PR #95818)

[David Green via llvm-commits]

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@@ -244,6 +244,9 @@ def FeatureExperimentalZeroingPseudos
 def FeatureUseScalarIncVL : SubtargetFeature<"use-scalar-inc-vl",
   "UseScalarIncVL", "true", "Prefer inc/dec over add+cnt">;
 
+def FeatureUseFixedIfEqualToScalable : SubtargetFeature<"use-fixed-if-equal-to-scalable",
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davemgreen wrote:

Subtarget features are considered the best way to add tuning features like this. There is even a comment about it
```
void AArch64Subtarget::initializeProperties(bool HasMinSize) {
  // Initialize CPU specific properties. We should add a tablegen feature for
  // this in the future so we can specify it together with the subtarget
  // features.
```

https://github.com/llvm/llvm-project/pull/95818