Linear-scan register allocation

Linear scan to assign physical registers. SSA liveness must have been computed already. It also spills registers that are live during gc calls.

Linear Scan Register Allocation by Massimiliano Poletto and Vivek Sarkar
Linear Scan Register Allocation for the Java HotSpot Client Compiler by Christian Wimmer and
Quality and Speed in Linear-scan Register Allocation by Omri Traub, Glenn Holloway, Michael D. Smith

Optimization pass entry point:
linear-scan ( cfg -- )