use a new Ref type for registers

This might not be a good idea, the problem was that
many spurious registers would be added to the Bits
data-structures during compilation (and would
always remain 0).  However, doing the above
modification de-uniformizes the handling of temps
and regs, this makes the code longer and not
really nicer.  Also, additional Bits structures
are required to track the registers independently.

Overall this might be a bad idea to revert.

lisc/Makefile

@@ -1,5 +1,5 @@
 BIN = lisc
-OBJ = main.o parse.o ssa.o live.o # isel.o spill.o rega.o emit.o
+OBJ = parse.o ssa.o live.o isel.o spill.o rega.o main.o # emit.o main.o
 
 CFLAGS = -Wall -Wextra -std=c11 -g -pedantic