Make regalloc2 panic-clean: always return errors when impossible constraints occur

Right now, the allocator can panic if the client provides impossible constraints, such as requiring two different operands to be placed in the same `PReg`.

While this represents a programming error in the client and *shouldn't* arise from an invalid input program, it is always better to bubble up errors; we should reserve `panic!()` and `assert!()` for conditions that can only be violated due to errors in regalloc2 itself.