Support --split-input-file

Especially for "negative" tests (invalid IR triggering verifiers, etc), it quickly becomes rather tedious to separate everything into its own *.mlir file. `mlir-opt` handles this with `--split-input-file`, which allows you to have multiple failing test in a single *.mlir file.

The issue names no implementation file, test, or entry point. Start by locating the command-line handling and input parsing related to mlir-opt-style options, then compare the requested --split-input-file behavior with mlir-opt. Done means multiple failing tests can be kept in one .mlir file and the option works as described.

Written by the indexing model from the issue text.