Handles joining of a jump instruction to its targets.
|:: (FR freeRegs, Instruction instr)|
|-> BlockMap RegSet|
maps the unique of the blockid to the set of vregs that are known to be live on the entry to each block.
id of the current block
branch instr on the end of the source block.
|-> RegM freeRegs ([NatBasicBlock instr], instr)|
For a jump instruction at the end of a block, generate fixup code so its vregs are in the correct regs for its destination.