[DebugInfo][RemoveDIs] Support finding DPValues like dbg.values (#71952)

This patch extends findDbgValue and friends to optionally fill out a vector
of DPValue pointers, containing DPValues that refer to the sought Value.
This will allow us to incrementally add instrumentation to other
optimisation passes one-at-a-time, while un-instrumented passes will not
(yet) update DPValues.

Unit tests to check this behaves in the same way as dbg.values.