[mlir][vector][nfc] Add tests + update docs for narrow-type emulation (#115460)
The documentation for narrow-type emulation was sparse, so I’ve expanded
it with additional clarifications (e.g., specifying that the example
discusses `i4` -> `i8` emulation).
I also noticed some inconsistencies in testing for narrow-type
emulation, with several cases covered only for "loading" and missing for
"storing." To address this, I’ve:
* Added comments in the test file for easier reference,
* Added the missing tests for `vector.maskedstore`.
Additionally, I’ve renamed tests for `vector.masked{load|store}` for
clarity:
* `@vector_cst_maskedload_i8` -> `@vector_maskedload_i8_constant_mask`.
This makes it easier to contrast with similar functions, such as
`@vector_maskedload_i8`.
Lastly, I’ve added a high-level comment in VectorEmulateNarrowType.cpp
to clarify the overall design and intent of the file.