The arguments to the entry block arrive in registers determined by the ABI. This information is stored in the signature. Use a separate function for coloring entry block arguments using the signature information. We can't handle stack arguments yet.
This gives the target ISA a chance to reserve registers like the stack pointer or hard-wired 0 registers like %x0 on RISC-V.
RISC architectures that take a return address in a register can use a special-purpose `link` return value to do so.
Run functions through the register allocator, and then filecheck.
