[DebugInfo][RemoveDIs] Instrument MergeFunctions for DPValues (#80974)

The MergeFunctions pass has a "preserve some debug-info" mode that tries
to preserve parameter values. This patch generalises its decision-making
so that it applies to both debug-info stored in intrinsics, and
debug-info stored in DPValue objects. For the most part this involves
using a generic lambda and applying it to each type of object.

(Normally we avoid debug-info affecting the code generated, but this is
hidden behind a command line switch, so won't usually be encountered by
users).

Note that this diff is messy, but that's because I'm hoisting some code
into lambdas. The actual decision making processes here are identical.