Add some top-down timing measurements to compilation

Somewhat confusing, since us_compiling_analysis_liveness is
also contained in us_compiling_irgen, and I'm not sure of a good
way to avoid that for now.

But anyway, for "import re", the breakdown is roughly:
- 215ms definedness analysis
- 50ms liveness analysis
- 80ms phi analysis
- 200ms type analysis
- 5ms irgen
- 25ms other compilation stuff
- 40ms non-compilation time