[mlir][transform] SequenceOp: Top-level operations can be used as matchers

As a convenience to the user, top-level sequence ops can optionally be used as matchers: the op type is specified by the type of the block argument.

This is similar to how pass pipeline targets can be specified on the command line (`-pass-pipeline='builtin.module(func.func(...))`).

Differential Revision: https://reviews.llvm.org/D153121