Simplify use of mir_opt_level

Simplify use of mir_opt_level

Remove the unused top level option by the same name, and retain the
debug option.

Use -Zmir-opt-level=1 as default.

One pass is enabled by default but wants to be optional:

- Instcombine requires mir_opt_level > 0

Copy propagation is not used by default, but used to be activated by
explicit -Zmir-opt-level=1. It must move one higher to be off by
default:

- CopyPropagation requires mir_opt_level > 1

Deaggregate is not used by default, and used to be on a different level
than CopyPropagation:

- Deaggreate requires mir_opt_level > 2