[RemoveDIs][DebugInfo] Check for null marker when printing DPValues (#82238)

The function to print DPValues currently tries to incorporate the
function it is part of, which is found through its marker; this means
when we try to print a DPValue with no marker, we dereference a nullptr.
We can print instructions without parents, and so the same should be
true for DPValues; this patch changes DPValue::print to check for a null
marker and avoid dereferencing it.

Fixes issue: https://github.com/llvm/llvm-project/issues/82230