Merge pull request #24925 from fengyuentau:loongarch_handle_warnings

Handle warnings in loongson-related code #24925

See https://github.com/fengyuentau/opencv/actions/runs/7665377694/job/20891162958#step:14:16

Warnings needs to be handled before we add the loongson server to our CI.

### Pull Request Readiness Checklist

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

- [x] I agree to contribute to the project under Apache 2 License.
- [x] 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
- [x] The PR is proposed to the proper branch
- [x] There is a reference to the original bug report and related work
- [x] 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

modules/core/include/opencv2/core/hal/intrin_lsx.hpp

@@ -860,6 +860,7 @@ class v_lsx_palignr_u8_class<imm, false, true, false, false, false>
 public:
     inline __m128i operator()(const __m128i& a, const __m128i& b) const
     {
+        CV_UNUSED(b);
         return a;
     }
 };
@@ -880,6 +881,7 @@ class v_lsx_palignr_u8_class<imm, false, false, false, true, false>
 public:
     inline __m128i operator()(const __m128i& a, const __m128i& b) const
     {
+        CV_UNUSED(a);
         return b;
     }
 };

modules/dnn/src/layers/cpu_kernels/fast_gemm_kernels.simd.hpp

@@ -313,8 +313,7 @@ static inline void fast_gemm12x16_f32(int k, const char *a_, const char *b_, cha
     const float* b = (const float*)b_;
     float* c = (float*)c_;
 
-    __m256i dummy;
-    __m256 s00  = (__m256)__lasx_xvxor_v(dummy, dummy), s01  = s00,
+    __m256 s00  = _v256_setall_ps(0), s01  = s00,
            s10  = s00, s11  = s00,
            s20  = s00, s21  = s00,
            s30  = s00, s31  = s00,