6994630: java/lang/instrument/IsModifiableClassAgent.java fails with -XX:+EnableInvokeDynamic
Summary: The logic of ClassFileParser::java_dyn_MethodHandle_fix_pre needs to take care of an already changed vmentry signature.
Reviewed-by:

There is a problem with the logic of
ClassFileParser::java_dyn_MethodHandle_fix_pre. The method searches
for a field called "vmentry" with the signature "byte". The signature
of the field is then changed to vmSymbols::machine_word_signature() to
hold an unmanged pointer.

When the method is called again the field's signature is already
changed and it throws a VirtualMachineError resulting in a
VerifyError.

The fix is to also check for the field with the already changed
signature.

Tested with the failing testcase.

I also changed the type of the fields argument since we don't update
the field pointer and removed a duplicated comment.