[x64] Word32 comparisons automatically truncate Word64 inputs.

On Intel platforms, the kX64Cmp32 and kX64Test32 operations in
TurboFan's backend automatically truncate their inputs to Word32
(aka they don't look at the upper bits), so the instruction selection
can silently ignore TruncateInt64ToInt32 on the inputs.

Bug: v8:8178
Change-Id: Ia50a38cac927e5b2155f092a8885da255a3dddca
Reviewed-on: https://chromium-review.googlesource.com/1227935Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55968}

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

@@ -1667,6 +1667,17 @@ void VisitWordCompare(InstructionSelector* selector, Node* node,
   Node* left = node->InputAt(0);
   Node* right = node->InputAt(1);
 
+  // The 32-bit comparisons automatically truncate Word64
+  // values to Word32 range, no need to do that explicitly.
+  if (opcode == kX64Cmp32 || opcode == kX64Test32) {
+    while (left->opcode() == IrOpcode::kTruncateInt64ToInt32) {
+      left = left->InputAt(0);
+    }
+    while (right->opcode() == IrOpcode::kTruncateInt64ToInt32) {
+      right = right->InputAt(0);
+    }
+  }
+
   opcode = TryNarrowOpcodeSize(opcode, left, right, cont);
 
   // If one of the two inputs is an immediate, make sure it's on the right, or