[mlir] Add `test-convergence` option to Canonicalizer tests

[mlir] Add `test-convergence` option to Canonicalizer tests

This new option is set to `false` by default. It should  be set only in Canonicalizer tests to detect faulty canonicalization patterns. I.e., patterns that prevent the canonicalizer from converging. The canonicalizer should always convergence on such small unit tests that we have in `canonicalize.mlir`.

Two faulty canonicalization patterns were detected and fixed with this change.

Differential Revision: https://reviews.llvm.org/D140873