[SelectionDAG][DebugInfo][RemoveDIs] Handle entry value variables in DPValues too (#78726)

This patch abstracts visitEntryValueDbgValue to deal with the substance
of variable locations (Value, Var, Expr, DebugLoc) rather than how
they're stored. That allows us to call it from handleDebugValue, which
is similarly abstracted. This allows the entry-value behaviour (see the
test) to be supported with non-instruction debug-info too!.