[DebugInfo][RemoveDIs] Implement redundant elimination for DPValues (#72284)

This pass steps through a block forwards and backwards, identifying those
variable assignment records that are redundant, and erases them, saving us
a decent wedge of compile-time. This patch re-implements it to use the
replacement for DbgValueInsts, DPValues, in an almost identical way.

Alas the test I've added the try-remove-dis flag to is the only one I've
been able to find that manually runs this pass.