[DirectX] Make DXILOpLowering responsible for cleaning up dead intrinsics (#138199)

This moves the responsibility for cleaning up dead intrinsics from
DXILFinalizeLinkage to DXILOpLowering, and moves DXILFinalizeLinkage
back to it's pre-#136244 place in the pipeline. Doing this avoids issues
with DXIL passes running on obviously dead code, and makes it more clear
what DXILFinalizeLinkage is really doing.

This also helps with the story for #134260, as cleaning up dead
intrinsics doesn't make sense if this becomes a more generic pass.

Note that test/CodeGen/DirectX/remove-dead-intriniscs.ll already covers
most of the testing here. It'd be nice to have something that catches
the regression from changing the pass ordering but I couldn't come up
with anything that wouldn't be incredibly fragile.

Fixes #138180.

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