[Turbofan] Make the -0 deopt case more efficient in multiplication.

src/compiler/machine-operator-reducer.cc

Index: src/compiler/machine-operator-reducer.cc
diff --git a/src/compiler/machine-operator-reducer.cc b/src/compiler/machine-operator-reducer.cc
index fd402ee1c7fecbea37ff222c6d57d9d188eb343a..4e2a3777b1d9b7ea162533119ad7229427ff9477 100644
--- a/src/compiler/machine-operator-reducer.cc
+++ b/src/compiler/machine-operator-reducer.cc
@@ -264,6 +264,16 @@ Reduction MachineOperatorReducer::Reduce(Node* node) {
         return ReplaceBool(m.left().Value() < m.right().Value());
       }
       if (m.LeftEqualsRight()) return ReplaceBool(false);  // x < x => false
+      if (m.left().IsWord32Or() && m.right().Is(0)) {      // (X | Y) < 0 ...
+        // This targets the check for -0 after multiplication.

[Benedikt Meurer, 2016/07/20 17:37:44]

This is a detail that is not necessarily relevant. I'd prefer to have a single
comment like this instead:

// (x | K) < 0 => true or (K | x) < 0 => true iff K < 0

+        Int32BinopMatcher mleftmatcher(m.left().node());
+        if ((mleftmatcher.left().HasValue() &&

[Benedikt Meurer, 2016/07/20 17:37:44]

Please add a IsNegative() helper to the Int32Matcher (similar to what we have
for Float64Matcher already) and use that here for left and right checks.

+             mleftmatcher.left().Value() < 0) ||
+            (mleftmatcher.right().HasValue() &&
+             mleftmatcher.right().Value() < 0)) {
+          return ReplaceBool(true);  // and X < 0 or Y < 0 => true
+        }
+      }
       break;
     }
     case IrOpcode::kInt32LessThanOrEqual: {