value
joinToTargets :: (FR freeRegs, Instruction instr)=> BlockMap RegSetmaps the unique of the blockid to the set of vregs that are known to be live on the entry to each block.
-> BlockIdid of the current block
-> instrbranch 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.