[DebugInfo][RemoveDIs] Prevent duplicate DPValues from being returned by findDbgIntrinsics (#82764)

Fixes the error described here:
https://github.com/llvm/llvm-project/commit/a93a4ec7dd205b965ee5597314bb376520cd736c#commitcomment-138965199

The function `findDbgIntrinsics` is used to return a list of debug
intrinsics and DPValues that use a given value, with the intent that no
duplicates are returned in either list. For DPValues, we've guarded
against DPValues that use a value multiple times as part of a DIArgList,
but we have not guarded against DPValues that use a value multiple times
as separate operands (currently only possible for `dbg_assign`s,
something I missed in my implementation of that type!). This patch adds
a guard, and also updates a test to cover this case.