[DebugInfo][RemoveDIs] Handle DPValues in SelectOptimize (#79005)

When there are debug intrinsics in-between groups of select
instructions, select-optimise sinks them into the "end" block. This
needs to be replicated for DPValues, the non-instruction variable
assignment object. Implement that and add a RUN line to a test that was
sensitive to this to ensure it gets tested.

(The exact range of instructions being transformed here is a little
fiddly, hence I've gone with a helper lambda).