RFR: Streamline write-barrier-prologue (C2 part)

[Roman Kennke]

Thank you!

I verified that this works correctly. I'll run some benchmarks later.
From earlier experiments, I strongly suspect that it will perform equal
or better compared to what we currently have.

Assuming benchmark results come out good, is the following patch ok to
push? It's my patch plus your null-checking patch:

http://cr.openjdk.java.net/~rkennke/streamline-wb-prologue/webrev.02/

Roman