JIT memory statistic, introduced with JDK-8317683, shows memory usage by compilation. That is useful.

We can build on that by:
- adding a way to see which C2 compiler phases were actually allocating or releasing memory, and when
- when reaching a peak (and then, e.g., asserting since we hit the MemLimit), show a breakup of the total memory by phase. That makes analysis of high memory situations much more fluid
- we also could use a finer-granular arena statistics