RFR(S): 8233081: C1: PatchingStub for field access copies too much

[Doerr, Martin]

Hi,

I'd like to fix an issue in C1's PatchingStub implementation for "access_field_id".
We had noticed that the code in the template exceeded the 255 byte limitation when switching on VerifyOops on PPC64.
I'd like to improve the situation for all platforms.

More detailed bug description:
https://bugs.openjdk.java.net/browse/JDK-8233081

I need a function to determine how many bytes are needed for the NativeMovRegMem.
x86 has next_instruction_address() which could in theory be used, but I noticed that it's dead code which is no longer correct.
Is it ok to remove it?
I'd also like to remove the constant instruction_size from NativeMovRegMem because it's not constant.
I'd prefer to introduce num_bytes_to_end_of_patch() for the purpose of determining how many bytes to copy for the "access_field_id" PatchingStub.
We can factor out the offset computation from offset() and set_offset() and reuse it. This enforces consistency.

Webrev:
http://cr.openjdk.java.net/~mdoerr/8233081_C1_access_field_patching/webrev.00/

Please review.

Best regards,
Martin