Refactor LowerVectorTransfersPass using pattern rewriters

This introduces a generic lowering pass for ML functions.  The pass is
parameterized by template arguments defining individual pattern rewriters.
Concrete lowering passes define individual pattern rewriters and inherit from
the generic class that takes care of allocating rewriters, traversing ML
functions and performing the actual rewrite.

While this is similar to the greedy pattern rewriter available in
Transform/Utils, it requires adjustments due to the ML/CFG duality.  In
particular, ML function rewriters must be able to create statements, not only
operations, and need access to an MLFuncBuilder.  When we move to using the
unified function type, the ML-specific rewriting will become unnecessary.

Use LowerVectorTransfers as a testbed for the generic pass.

PiperOrigin-RevId: 225887424