[VectorUtils] Use early_inc_range instead of DelSet (NFC).

DelSet was used to avoid invalidating the current iterator while
modifying the map we are iterating over.

By using an early_inc_range, (which increments to iterator 'early',
allowing us to remove the current element), we can get rid of DelSet.

Reviewers: gilr, rengolin, Ayal, hsaito

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D78420