proc_rmdead: use explicit pattern set when there are no wildcards

proc_rmdead: use explicit pattern set when there are no wildcards

If width of a case expression was large, explicit patterns could cause
the existing logic to take an extremely long time, or exhaust the
maximum size of the underlying set. For cases where all of the patterns
are fully defined and there are no constants in the case expression,
this change uses a simple set to track which patterns have been seen.