[tablegen] Delete duplicates from a vector without skipping elements

Tablegen's -gen-instr-info pass has a bug in its emitEnums() routine.
The function intends for values in a vector to be deduplicated, but it
accidentally skips over elements after performing a deletion.

I think there are smarter ways of doing this deduplication, but we can
do that in a follow-up commit if there's interest. See the thread:
[PATCH] TableGen InstrMapping Bug fix.

Patch by Tyler Kenney!

llvm-svn: 288408