Allow using more assignment operators on matrixes in shaders

(cherry picked from commit 0f42a29864336bf76df1418a7d8cc773795eff68)

servers/visual/shader_language.cpp

@@ -1252,7 +1252,7 @@ bool ShaderLanguage::_validate_operator(OperatorNode *p_op, DataType *r_ret_type
 			DataType nb = p_op->arguments[1]->get_datatype();
 
 			if (na == nb) {
-				valid = (na > TYPE_BOOL && na < TYPE_MAT2) || (p_op->op == OP_ASSIGN_MUL && na >= TYPE_MAT2 && na <= TYPE_MAT4);
+				valid = (na > TYPE_BOOL && na <= TYPE_MAT4);
 				ret_type = na;
 			} else if (na == TYPE_IVEC2 && nb == TYPE_INT) {
 				valid = true;