[RemoveDIs] Fix findDbgValues to return dbg_assign records too (#90471)

In the debug intrinsic class heirachy, a dbg.assign is a (inherits from)
dbg.value, so `findDbgValues` returns dbg.values and dbg.assigns (by
design). That hierarchy doesn't exist for DbgRecords - fix findDbgValues
to return dbg_assign records as well as dbg_values and add unittest.