We are in the process of adding more specific optimization functions to the RVV HAL, meanwhile, we took a look back and fixed some bugs in mean and merge:

• [merge] Fix stride mismatch in vector store segment instructions, and add tests for it.
  We previously only considered the cn is 2,3,4, and it is actually wrong when cn>4 (this patch also adds vectorization for the part where cn>4)

# The following are the newly added tests that have been fixed
[==========] 32 tests from 1 test case ran. (4 ms total)
[  PASSED  ] 24 tests.
[  FAILED  ] 8 tests, listed below:
[  FAILED  ] Core_Mat_arrays.SplitMerge/16, where GetParam() = (5, 8UC1)
[  FAILED  ] Core_Mat_arrays.SplitMerge/17, where GetParam() = (5, 16UC1)
[  FAILED  ] Core_Mat_arrays.SplitMerge/20, where GetParam() = (6, 8UC1)
[  FAILED  ] Core_Mat_arrays.SplitMerge/21, where GetParam() = (6, 16UC1)
[  FAILED  ] Core_Mat_arrays.SplitMerge/24, where GetParam() = (7, 8UC1)
[  FAILED  ] Core_Mat_arrays.SplitMerge/25, where GetParam() = (7, 16UC1)
[  FAILED  ] Core_Mat_arrays.SplitMerge/28, where GetParam() = (8, 8UC1)
[  FAILED  ] Core_Mat_arrays.SplitMerge/29, where GetParam() = (8, 16UC1)

• [merge] Use static instead of inline to suppress some warnings.
• [mean] Fix wrong number of elements in 8UC4 with mask: nz should obviously be divided by 4 at the end, not in the loop.

Pull Request Readiness Checklist

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

• I agree to contribute to the project under Apache 2 License.
• To the best of my knowledge, the proposed patch is not based on a code under GPL or another license that is incompatible with OpenCV
• The PR is proposed to the proper branch
• There is a reference to the original bug report and related work
• There is accuracy test, performance test and test data in opencv_extra repository, if applicable
  Patch to opencv_extra has the same branch name.
• The feature is well documented and sample code can be built with the project CMake