cleanup in preprocessing dir

[IanWood1]

          nit: already in the `iree_compiler` namespace, can simplify this?

    : public Preprocessing::impl::ApplyPDLLPatternsPassBase<

I see a bunch of these public iree_compiler:: in the Preprocessing/ folder, could clean those up too (in a separate PR to keep this one focused)

Originally posted by ScottTodd in https://github.com/iree-org/iree/pull/17144#discussion_r1578436244

[benvanik]

Would be good cleanup! The style we hold is that iree_compiler can be omitted because all IREE code is in it, and then any dialect references require explicit specification (so "IREE::Flow::TensorReshapeOp", not "iree_compiler::IREE::Flow::TensorReshapeOp", "Flow::TensorReshapeOp", any using namespace of IREE-prefixed namespaces, etc). I've been trying to clean some of these up as I find them but more help would be appreciated!