profiling go

[Stephen Weeks]

> The best solution, IMHO, would really be to change the CPS->Machine
> register allocator so that at each limitCheck, farJump, and prim that
> enters the runtime, it generates a mapping from pseudo-regs to stack
> slots.  That way, everything is in pseudo-regs and the codegen would take
> care of moving values to the stack at the right times.  (This would be
> nice for limitChecks, I think, because we could set it up so that the
> moves occured after the frontier check; if you didn't need to GC, you
> wouldn't pay for setting up the stack.)

I like this idea.  We need the mapping at nontail calls too.  This seems almost
too easy!