[mlir][NFC] Make test-lower-to-llvm a named pipeline

This patch changes the `test-lower-to-llvm` pass into a named pipeline.
The functionality is unchanged but thanks to this change, we don't have
to pull the dependencies of all the passes that this pass calls.
In other words, `TestLowerToLLVMPass::getDependDialects` was supposed to
transitively declare all the dialects that were used in the union of all
the invoked passes.

NFC

Differential Revision: https://reviews.llvm.org/D139840