emit_x64_vector: Add break to final case in EmitVectorRoundingHalvingAddUnsigned()

This doesn't alter behavior but does make the code better if anything
else is ever added to this function in the future.

src/backend_x64/emit_x64_vector.cpp

@@ -1872,6 +1872,7 @@ static void EmitVectorRoundingHalvingAddUnsigned(size_t esize, EmitContext& ctx,
         code.paddd(a, b);
 
         ctx.reg_alloc.DefineValue(inst, a);
+        break;
     }
     }
 }