Investiage increased heap size with stress GC

[qinsoon]

The min heap size for the compiler inference benchmark is 1090MB. We run the benchmark with 1200MB heap size. However, if we use stress factor to trigger stress GC frequently (such as MMTK_STRESS_FACTOR=52428800, trigger a stress GC every 50MB), we see the benchmark run OOM with 1200MB heap, and 2000MB heap.

We need to investigate why this happens.

One thing that might be related is the overflow alloc, which always uses clean blocks.