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

[David Sherwood 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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david-arm wrote:

Does this need to be a full-blown feature if you intend to just enable it by default for V2 anyway? When we added VScaleForTuning, we just used a boolean in AArch64Subtarget that gets set during initialisation for the particular CPU. See AArch64Subtarget::initializeProperties.

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