[DebugInfo][RemoveDIs] Interpret DPValue objects in SelectionDAG (#72253)

DPValues are the non-intrinsic replacements for dbg.values, and when an
IR function is converted by SelectionDAG we need to convert the variable
location information in the same way. Happily all the information is in
the same format, it's just stored in a slightly different object,
therefore this patch refactors a few things to store the set of
{Variable,Expr,DILocation,Location} instead of just a pointer to a
DbgValueInst.

This also adds a hook in llc that's much like the one I've added to opt
in PR #71937, allowing tests to optionally ask for the use RemoveDIs
mode if support for it is built into the compiler.

I've added that flag to a variety of SelectionDAG debug-info tests to
ensure that we get some coverage on the RemoveDIs / debug-info-iterator
buildbot.