[simd_neon]refine the S_to_bits to improve "find_first/last_set" for neon

find_first_set and find_last_set method is not optimal for neon,
it need to be improved by synthesized with horizontal adds(vaddv)
which will reduce the generated assembly code; in the following cases,
vaddvq_s16 will generate 2 instructions but vpadd_s16 will generate 4
instrunctions:
```
 # vaddvq_s16
    vaddvq_s16(__asint);
    //  addv    h0, v1.8h
    //  smov    w1, v0.h[0]
 # vpadd_s16
   vpaddq_s16(vpaddq_s16(vpaddq_s16(__asint, __zero), __zero), __zero)[0]
   // addp v1.8h,v1.8h,v2.8h
   // addp v1.8h,v1.8h,v2.8h
   // addp v1.8h,v1.8h,v2.8h
   // smov    w1, v1.h[0]
 #
```

Author: yaozhongxiao

experimental/bits/simd_neon.h

@@ -314,8 +314,7 @@ struct _MaskImplNeonMixin
 		  });
 	      __asint &= __bitsel;
 #ifdef __aarch64__
-	      return vpaddq_s16(vpaddq_s16(vpaddq_s16(__asint, __zero), __zero),
-				__zero)[0];
+	      return vaddvq_s16(__asint);
 #else
 	      return vpadd_s16(
 		vpadd_s16(vpadd_s16(__lo64(__asint), __hi64(__asint)), __zero),
@@ -331,7 +330,7 @@ struct _MaskImplNeonMixin
 		  });
 	      __asint &= __bitsel;
 #ifdef __aarch64__
-	      return vpaddq_s32(vpaddq_s32(__asint, __zero), __zero)[0];
+	      return vaddvq_s32(__asint);
 #else
 	      return vpadd_s32(vpadd_s32(__lo64(__asint), __hi64(__asint)),
 			       __zero)[0];
@@ -354,8 +353,12 @@ struct _MaskImplNeonMixin
 		    return static_cast<_I>(__i < _Np ? 1 << __i : 0);
 		  });
 	      __asint &= __bitsel;
+#ifdef __aarch64__
+	      return vaddv_s8(__asint);
+#else
 	      return vpadd_s8(vpadd_s8(vpadd_s8(__asint, __zero), __zero),
 			      __zero)[0];
+#endif
 	    }
 	  else if constexpr (sizeof(_Tp) == 2)
 	    {
@@ -365,12 +368,20 @@ struct _MaskImplNeonMixin
 		    return static_cast<_I>(__i < _Np ? 1 << __i : 0);
 		  });
 	      __asint &= __bitsel;
+#ifdef __aarch64__
+	      return vaddv_s16(__asint);
+#else
 	      return vpadd_s16(vpadd_s16(__asint, __zero), __zero)[0];
+#endif
 	    }
 	  else if constexpr (sizeof(_Tp) == 4)
 	    {
 	      __asint &= __make_vector<_I>(0x1, 0x2);
+#ifdef __aarch64__
+	      return vaddv_s32(__asint);
+#else
 	      return vpadd_s32(__asint, __zero)[0];
+#endif
 	    }
 	  else
 	    __assert_unreachable<_Tp>();