CTZ instruction implemented as optional operator.

src/compiler/machine-operator.h

Index: src/compiler/machine-operator.h
diff --git a/src/compiler/machine-operator.h b/src/compiler/machine-operator.h
index 27abfb4accca6e52ecbfe5c481fe82256a2d8402..abb2d045be1bdbd18850892458db14128b454ceb 100644
--- a/src/compiler/machine-operator.h
+++ b/src/compiler/machine-operator.h
@@ -117,9 +117,10 @@ class MachineOperatorBuilder final : public ZoneObject {
     kInt32DivIsSafe = 1u << 7,
     kUint32DivIsSafe = 1u << 8,
     kWord32ShiftIsSafe = 1u << 9,
+    kWord32Ctz = 1u << 10,
     kAllOptionalOps = kFloat32Max | kFloat32Min | kFloat64Max | kFloat64Min |
                       kFloat64RoundDown | kFloat64RoundTruncate |
-                      kFloat64RoundTiesAway
+                      kFloat64RoundTiesAway | kWord32Ctz
   };
   typedef base::Flags<Flag, unsigned> Flags;
 
@@ -135,6 +136,7 @@ class MachineOperatorBuilder final : public ZoneObject {
   const Operator* Word32Ror();
   const Operator* Word32Equal();
   const Operator* Word32Clz();
+  const OptionalOperator Word32Ctz();
   bool Word32ShiftIsSafe() const { return flags_ & kWord32ShiftIsSafe; }
 
   const Operator* Word64And();