Add comment about building loop tree.
Sparse Conditional Constant Propagation: constant propagation and removing death paths of CFG
Machinize: Machine-dependent code (e.g. in mir-gen-x86_64.c)
transforming MIR for calls ABI, 2-op insns, etc.
+ Finding Loops: Building loop tree which is used in subsequent register allocation.
Building Live Info: Calculating live in and live out for the basic blocks.
Build Live Ranges: Calculating program point ranges for registers.
Assign: Priority-based assigning hard regs and stack slots to registers.
