[crankshaft] Re-add kAllowUndefinedAsNaN flag for bitwise binary ops

It appears to have been dropped accidentally as part of 1150092b's
removal of strong mode for binary ops.

Review-Url: https://codereview.chromium.org/2092493002
Cr-Commit-Position: refs/heads/master@{#37229}

src/crankshaft/hydrogen-instructions.h

@@ -3740,6 +3740,7 @@ class HBitwiseBinaryOperation : public HBinaryOperation {
       : HBinaryOperation(context, left, right, type) {
     SetFlag(kFlexibleRepresentation);
     SetFlag(kTruncatingToInt32);
+    SetFlag(kAllowUndefinedAsNaN);
     SetAllSideEffects();
   }