[MetaRenamer] Rename only unnamed instructions in mode renaming instructions

6f9e743b introduced a mode which renames only instructions in
the function. This change updates that mode to skip instructions that are already named.
This serves the original purpose of the mode (rename-only-inst) which is:
1. Modify IR without failing verifier with serially ordered number
   requirement (%1, %2, %3 required in order).
2. Give meaningful names to instructions.