[mlir][spirv] Support `vector.step` in vector to spirv conversion

[kuhar]

The vector.step op was added in #96776.

We won't need integration tests for this, just LIT will do.

[kuhar]

@Angelz913

[llvmbot]

@llvm/issue-subscribers-mlir-spirv

Author: Jakub Kuderski (kuhar)

The `vector.step` op was added in https://github.com//pull/96776.

We won't need integration tests for this, just LIT will do.