Add option to enable MIR inlining independently of mir-opt-level
Add `-Zinline-mir` option that enables MIR inlining independently of the
current MIR opt level. The primary use-case is enabling MIR inlining on the
default MIR opt level.
Turn inlining thresholds into optional values to make it possible to configure
different defaults depending on the current mir-opt-level (although thresholds
are yet to be used in such a manner).
ae5e024194