[NFC] Return unique dbg intrinsics from findDbgValues and findDbgUsers

The out-param vector from findDbgValues and findDbgUsers should not include
duplicates, which is possible if the debug intrinsic uses the value multiple
times. This filter is already in place for multiple uses in a `DIArgLists`;
extend it to cover dbg.assigns too because a Value may be used in both the
address and value components.

Additionally, refactor the duplicated functionality between findDbgValues and
FindDbgUsers into a new function findDbgIntrinsics.

Reviewed By: jmorse, StephenTozer

Differential Revision: https://reviews.llvm.org/D148788