[turbofan,x64] Replace Int32Sub(x,0) with EmitIdentity

Address a TODO: Use EmitIdentity instead of kArchNop.

Bug: v8:7947
Change-Id: Idd7de1bcffaf56f7eaf5d65be4dae3257d085ea8
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1674031Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62338}

src/compiler/backend/x64/instruction-selector-x64.cc

@@ -907,9 +907,9 @@ void InstructionSelector::VisitInt32Sub(Node* node) {
   if (m.left().Is(0)) {
     Emit(kX64Neg32, g.DefineSameAsFirst(node), g.UseRegister(m.right().node()));
   } else if (m.right().Is(0)) {
-    // TODO(jarin): We should be able to use {EmitIdentity} here
-    // (https://crbug.com/v8/7947).
-    Emit(kArchNop, g.DefineSameAsFirst(node), g.Use(m.left().node()));
+    // {EmitIdentity} reuses the virtual register of the first input
+    // for the output. This is exactly what we want here.
+    EmitIdentity(node);
   } else if (m.right().HasValue() && g.CanBeImmediate(m.right().node())) {
     // Turn subtractions of constant values into immediate "leal" instructions
     // by negating the value.