Previously it was:
tier 0: ast interpreter
tier 1: llvm, no speculations, no llvm opts
tier 2: llvm, w/ speculations, no llvm opts
tier 3: llvm, w/ speculations, w/ llvm opts

tier 2 seemed pretty useless, and very little would stay in it.  Also,
OSR would always skip from tier 1 to tier 3.

Separately, add configurable OSR/reopt thresholds.  This is mostly for the
sake of tests, where we can set lower limits and force OSR/reopts to happen.