Clean up CompiledMethod::oops_reloc_begin()

[Erik Österlund]

Hi,

In CompiledMethod::oops_reloc_begin() there is code to prevent looking 
for oops at the verified entry of an nmethod that is not entrant, as a 
native jump instruction has been overwritten there. Except there would 
*never* be any immediate oops there for any compiler, even before 
becoming not entrant, with or without OSR compilation, so this special 
handling of not entrant vs entrant nmethods is seemingly completely 
unnecessary.

This gets increasingly awkward when oops_do is called concurrently with 
concurrent class unloading, where an nmethod is racing to become not 
entrant.

To clean this up, I propose to change the boundary for immediate oop 
scanning and start looking for oops only after the frame building is 
completed, as there is absolutely no point in looking for oops before 
that in the first place. This removes unnecessary workarounds that exist 
today, and removes unnecessary races going forward.

It seems like Graal as JIT also inlines oops into the code stream, but 
never sets the CodeOffsets::Frame_Complete in the nmethod. So I 
recognize such nmethods and unconditionally start scanning at the 
verified entry + native jump instruction size. I spoke to the Graal 
folks, and they said they do not put oops in there either. So I think 
everyone involved should be happy with this solution.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8212585

Webrev:
http://cr.openjdk.java.net/~eosterlund/8212585/webrev.00/

Thanks,
/Erik