[llvm-reduce][DebugInfo] Support reducing non-instruction debug-info (#78995)

LLVM will shortly be able to represent variable locations without
encoding information into intrinsics -- they'll be stored as DPValue
objects instead. We'll still need to be able to llvm-reduce these
variable location assignments just like we can with intrinsics today,
thus, here's an llvm-reduce pass that enumerates and reduces the DPValue
objects.

The test for this is paradoxically written with dbg.value intrinsics:
this is because we're changing all the core parts of LLVM to support
this first, with the textual IR format coming last. Until that arrives,
testing the llvm-reduce'ing of DPValues needs the added test using
intrinsics. We should be able to drop the variable assignment using
%alsoloaded using this method. As with the other llvm-reduce tests, I've
got one set of check lines for making the reduction happen as desired,
and the other set to check the final output.