Maintain invariant: MVars on mut_list are dirty

The fix for 18919 was somewhat incomplete: while the MVars were
correctly added to the mut_list via dirty_MVAR(), their info table
remained "clean".

While this is mostly harmless in non-debug builds, but trips an
assertion in the debug build, and may result in the MVar being
needlessly being added to the mut_list multiple times.

Resolves: #19145