compiler/arm64: change nops to use EmitIdentity

This affects VisitChangeUint32ToUint64 and VisitTruncateInt64ToInt32.

The geometric mean of changes across octane, ares6-1.0.1, jetstream2,
and web-tooling-0.5.3, was an improvement of 1.2% when running on a
Cortex-A53.

Change-Id: Ib551641fb78ce4060100129e12f23cd02b0b3b27
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1669690Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62327}

src/compiler/backend/arm64/instruction-selector-arm64.cc

@@ -1599,7 +1599,7 @@ void InstructionSelector::VisitChangeUint32ToUint64(Node* node) {
       // 32-bit operations will write their result in a W register (implicitly
       // clearing the top 32-bit of the corresponding X register) so the
       // zero-extension is a no-op.
-      Emit(kArchNop, g.DefineSameAsFirst(node), g.Use(value));
+      EmitIdentity(node);
       return;
     }
     case IrOpcode::kLoad: {
@@ -1610,7 +1610,7 @@ void InstructionSelector::VisitChangeUint32ToUint64(Node* node) {
         case MachineRepresentation::kWord8:
         case MachineRepresentation::kWord16:
         case MachineRepresentation::kWord32:
-          Emit(kArchNop, g.DefineSameAsFirst(node), g.Use(value));
+          EmitIdentity(node);
           return;
         default:
           break;
@@ -1665,10 +1665,9 @@ void InstructionSelector::VisitChangeCompressedSignedToTaggedSigned(
 
 void InstructionSelector::VisitTruncateInt64ToInt32(Node* node) {
   Arm64OperandGenerator g(this);
-  Node* value = node->InputAt(0);
   // The top 32 bits in the 64-bit register will be undefined, and
   // must not be used by a dependent node.
-  Emit(kArchNop, g.DefineSameAsFirst(node), g.UseRegister(value));
+  EmitIdentity(node);
 }
 
 void InstructionSelector::VisitFloat64Mod(Node* node) {