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

[Roland Westrelin]

http://cr.openjdk.java.net/~roland/8231291/webrev.01/

As discussed before:

https://mail.openjdk.java.net/pipermail/hotspot-compiler-dev/2019-September/035094.html
https://mail.openjdk.java.net/pipermail/hotspot-compiler-dev/2019-November/036171.html

Fully unrolling loops early helps EA. The change to cfgnode.cpp is
required because full unroll sometimes needs peeling which may add a phi
between a memory access and its AddP, a pattern that EA doesn't
recognize.

Roland.