Dump graph at several intermediate steps of register allocation. This makes it easier to diagnose issues related to spilling, coalescing, "CISC" spilling conversion, etc. in C2.

Note: All new phases should also be added to the CompilePhase enum [1] of the IR framework.

[1] https://github.com/openjdk/jdk/blob/master/test/hotspot/jtreg/compiler/lib/ir_framework/CompilePhase.java