RFR(S): 8231291: C2: loop opts before EA should maximally unroll loops

[Roland Westrelin]

Thanks for reviewing this, Vladimir.

> cfgnode.cpp - should we also check for is_top() to set `doit = false` and bailout?

This:

http://cr.openjdk.java.net/~roland/8231291/webrev.02/

> Why you added check in must_be_not_null()? It is used only in library_call.cpp and should not relate to this changes. 
> Did you find some issues?

I hit a crash when doing some CTW testing because an argument to
must_be_not_null() was already known to be not null. I suppose it's
unrelated to that change and something changed in the libraries instead.

Roland.