Let NaN flow as double into HBranch

src/x64/lithium-x64.cc

Index: src/x64/lithium-x64.cc
diff --git a/src/x64/lithium-x64.cc b/src/x64/lithium-x64.cc
index 980cb2112825a5d2969a1ea024120946efbf3c72..19b4945e8cc3738218db4361dee7f34db283f638 100644
--- a/src/x64/lithium-x64.cc
+++ b/src/x64/lithium-x64.cc
@@ -1014,10 +1014,12 @@ LInstruction* LChunkBuilder::DoBranch(HBranch* instr) {
 
   LBranch* result = new(zone()) LBranch(UseRegister(value));
   // Tagged values that are not known smis or booleans require a
-  // deoptimization environment.
+  // deoptimization environment. Except in the Generic case.
+  ToBooleanStub::Types expected = instr->expected_input_types();
   Representation rep = value->representation();
   HType type = value->type();
-  if (rep.IsTagged() && !type.IsSmi() && !type.IsBoolean()) {
+  if (rep.IsTagged() && !type.IsSmi() && !type.IsBoolean() &&
+      !expected.IsGeneric()) {
     return AssignEnvironment(result);
   }
   return result;