[x64] Avoid movl before table jump when possible

If the input to a Switch happens to be something that we already know to
be zero-extended, then we don't need to emit a movl to zero-extend it.
This is a tiny optimization, but speeds up the Mono interpreter by 3%
because it's heavily dependent on switch dispatch speed.

Bug: v8:10606
Change-Id: I67ab6b6aeb93df4e420ea2afef41a2234d7da979
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2368195Reviewed-by: Zhi An Ng <zhin@chromium.org>
Commit-Queue: Seth Brenith <seth.brenith@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#69523}

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

@@ -2271,7 +2271,12 @@ void InstructionSelector::VisitSwitch(Node* node, const SwitchInfo& sw) {
              value_operand, g.TempImmediate(-sw.min_value()));
       } else {
         // Zero extend, because we use it as 64-bit index into the jump table.
-        Emit(kX64Movl, index_operand, value_operand);
+        if (ZeroExtendsWord32ToWord64(node->InputAt(0))) {
+          // Input value has already been zero-extended.
+          index_operand = value_operand;
+        } else {
+          Emit(kX64Movl, index_operand, value_operand);
+        }
       }
       // Generate a table lookup.
       return EmitTableSwitch(sw, index_operand);