VIL code generator has knowledge of x86 return register

[iafisher]

vil::Register::ret() returns a register with index 13, which maps to RAX in x86. There is actually no ret instruction in VIL; instead, the code generated for a return statement places the value of the expression in vil::Register::ret() and then jumps to the function's return label (so that every function has a single place of return).

The reason VIL has no ret instruction is that the x86 code generator needs to generate some code after the last instruction of the function but before the return to clean up the stack frame, but that would be awkward if ret were a regular instruction. Probably the solution is to move the stack clean-up logic in an abstracted form into VIL, and add a ret instruction to VIL that takes a register argument and let the x86 generator handle moving that register into RAX.