proc_prune: fix handling of exactly identical assigns.

proc_prune: fix handling of exactly identical assigns.

Before this commit, in a process like:
   process $proc$bug.v:8$3
     assign $foo \bar
     switch \sel
       case 1'1
         assign $foo 1'1
         assign $foo 1'1
       case
         assign $foo 1'0
     end
   end
both of the "assign $foo 1'1" would incorrectly be removed.

Fixes #1243.