[mlir][vector] Fix invalid IR in `ContractionOpLowering` (#78130)

If a rewrite pattern returns "failure", it must not have modified the
IR. This commit fixes
`Dialect/Vector/vector-contract-to-outerproduct-transforms-unsupported.mlir`
when running with `MLIR_ENABLE_EXPENSIVE_PATTERN_API_CHECKS`.

```
  * Pattern (anonymous namespace)::ContractionOpToOuterProductOpLowering : 'vector.contract -> ()' {
Trying to match "(anonymous namespace)::ContractionOpToOuterProductOpLowering"
    ** Insert  : 'vector.transpose'(0x5625b3a8cb30)
    ** Insert  : 'vector.transpose'(0x5625b3a8cbc0)
"(anonymous namespace)::ContractionOpToOuterProductOpLowering" result 0
  } -> failure : pattern failed to match
} -> failure : pattern failed to match

LLVM ERROR: pattern returned failure but IR did change
```

Note: `vector-contract-to-outerproduct-transforms-unsupported.mlir` is
merged into `vector-contract-to-outerproduct-matvec-transforms.mlir`.
The `greedy pattern application failed` error is not longer produced.
This error indicates that the greedy pattern rewrite did not
convergence; it does not mean that a pattern could not be applied.