[DebugInfo][RemoveDIs] Adjust AMDGPU passes to work with DPValues (#78736)

This patch tweaks two AMDGPU passes to use iterators rather than
instruction pointers for expressing an insertion point. This is needed
to accurately support DPValues, the non-instruction storage object for
debug-info.

Two tests were sensitive to this change (variable assignments were being
put in the wrong place), and I've added extra run-lines with the "try
new debug-info..." flag. These get tested on our public buildbot to
ensure they continue to work accurately.