[CostModel][AArch64] Model cost of extract.last.active intrinsic (clastb) (llvm#165739)

Adds some aarch64 cost model tests for extract.last.active, and produces
a lower cost when SVE is available (expecting clastb to be generated).

Author: huntergr-arm

llvm/lib/Target/AArch64/AArch64TargetTransformInfo.cpp

@@ -1032,6 +1032,13 @@ AArch64TTIImpl::getIntrinsicInstrCost(const IntrinsicCostAttributes &ICA,
     }
     break;
   }
+  case Intrinsic::experimental_vector_extract_last_active:
+    if (ST->isSVEorStreamingSVEAvailable()) {
+      auto [LegalCost, _] = getTypeLegalizationCost(ICA.getArgTypes()[0]);
+      // This should turn into chained clastb instructions.
+      return LegalCost;
+    }
+    break;
   default:
     break;
   }