RFR (S): G1: Fix bug with compressed oops on x86_64 and sparc

[Doerr, Martin]

Hi all,

we found a G1 bug in the template interpreter on x86_64 and sparc with compressed Oops.
"do_oop_store" performs a "store_heap_oop" which compresses the input register.
This compressed Oop is passed to the succeeding g1_write_barrier_post, which is wrong.
Correctly, g1_write_barrier_post needs the uncompressed Oop for the heap region crossing check.

This fix preserves the uncompressed Oop in a register which gets passed to the g1_write_barrier_post:
http://cr.openjdk.java.net/~goetz/webrevs/g1-cOops_bug/

Please supply a bug id and review this change.

Kind regards,
Martin

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