RFR: 8264358: Don't create invalid oop in method handle tracing

[Stefan Karlsson]

The `mh` field in:

struct MethodHandleStubArguments {
  const char* adaptername;
  oopDesc* mh;
  intptr_t* saved_regs;
  intptr_t* entry_sp;
};

doesn't always point to a valid object. The `oopDesc*` is then implicitly converted to an `oop` here:

void trace_method_handle_stub_wrapper(MethodHandleStubArguments* args) {
  trace_method_handle_stub(args->adaptername,
                           args->mh,
                           args->saved_regs,
                           args->entry_sp);
}

This gets caught by my ad-hoc verification code that verifies oops when they are created/used.

I propose that we don't create an oop until it `mh` is actually used, and it has been checked that the argument should contain a valid oop.  I started with a more elaborate fix that changed the type of `mh` to be `void*`, but then reverted to a more targetted fix to remove the early oopDesc* > oop conversion.

One thing that I am curious about is this code inside trace_method_handle_stub:
if (has_mh && oopDesc::is_oop(mh)) {
  mh->print_on(&ls);

Delaying the oopDesc* > oop conversion to after the `has_mh` check solves my verification failure, but I wonder if the `oopDesc::is_oop(mh)` call is really needed when we have the `has_mh` check?

-------------

Commit messages:
 - 8264358: Don't create invalid oop in method handle tracing

Changes: https://git.openjdk.java.net/jdk/pull/3242/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3242&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8264358
  Stats: 2 lines in 2 files changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3242.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3242/head:pull/3242

PR: https://git.openjdk.java.net/jdk/pull/3242