Remove spilling code from bir_passes when supported in constraint model

[ccasin]

bir_passes.ml has a function spill_hvars_and_adjust_stack that handles spilling for register-allocated variables that we know about because the user specified them. But really we need a more general notion of spilling that happens after register allocation and considers any live variable in a caller-saved register, not just the ones the user told us about. Once that's in the minizinc model (issue #16), remove spilling from bir_passes.