S390: InstanceOfStub incorrectly interprets the hole as a prototype.

Port 2aa070be

Original commit message:
    Repair this to match what the runtime correctly does, by first checking if the
    function is a constructor before we access the prototype.

R=mvstanton@chromium.org, joransiu@ca.ibm.com, michael_dawson@ca.ibm.com, mbrandy@us.ibm.com
BUG=

Review URL: https://codereview.chromium.org/1813003002

Cr-Commit-Position: refs/heads/master@{#34872}

src/s390/code-stubs-s390.cc

@@ -1414,8 +1414,12 @@ void InstanceOfStub::Generate(MacroAssembler* masm) {
   __ CompareObjectType(function, function_map, scratch, JS_FUNCTION_TYPE);
   __ bne(&slow_case);
 
-  // Ensure that {function} has an instance prototype.
+  // Go to the runtime if the function is not a constructor.
   __ LoadlB(scratch, FieldMemOperand(function_map, Map::kBitFieldOffset));
+  __ TestBit(scratch, Map::kIsConstructor, r0);
+  __ beq(&slow_case);
+
+  // Ensure that {function} has an instance prototype.
   __ TestBit(scratch, Map::kHasNonInstancePrototype, r0);
   __ bne(&slow_case);