Split spill_from() into spill_candidate() and spill_reg().
Split spill_from() into spill_candidate() and spill_reg().

We'll need to pick a spill candidate from a set and allow for the search
to fail to find anything.

This also allows slightly better panic messages when we run out of
registers.

sunfishcode/cretonne,stoklund/cretonne,sunfishcode/cretonne,sunfishcode/cretonne,stoklund/cretonne,stoklund/cretonne