[AArch64] [CostModel] Fix cost modelling for saturating arithmetic intrinsics (#152333)

The cost model previously overestimating throughput costs to wide
fixed-length saturating arithmetic intrinsics when using SVE with a
fixed vscale of 2. These costs ended up much higher than for the same
operations using NEON, despite being fully legal and efficient with SVE.
This patch adjusts the cost model to avoid penalising these intrinsics
under SVE.

Author: marykass-arm

llvm/lib/Target/AArch64/AArch64TargetTransformInfo.cpp

@@ -643,6 +643,13 @@ AArch64TTIImpl::getIntrinsicInstrCost(const IntrinsicCostAttributes &ICA,
         LT.second.getScalarSizeInBits() == RetTy->getScalarSizeInBits() ? 1 : 4;
     if (any_of(ValidSatTys, [&LT](MVT M) { return M == LT.second; }))
       return LT.first * Instrs;
+
+    TypeSize TS = getDataLayout().getTypeSizeInBits(RetTy);
+    uint64_t VectorSize = TS.getKnownMinValue();
+
+    if (ST->isSVEAvailable() && VectorSize >= 128 && isPowerOf2_64(VectorSize))
+      return LT.first * Instrs;
+
     break;
   }
   case Intrinsic::abs: {