ir_expression: Add static operator_string method

I've used this in quite a few debug commits that never reached an
up-stream tree.

src/glsl/ir.cpp

@@ -287,11 +287,15 @@ static const char *const operator_strs[] = {
    "pow",
 };
 
+const char *ir_expression::operator_string(ir_expression_operation op)
+{
+   assert((unsigned int) op < Elements(operator_strs));
+   return operator_strs[op];
+}
+
 const char *ir_expression::operator_string()
 {
-   assert((unsigned int) operation <=
-	  sizeof(operator_strs) / sizeof(operator_strs[0]));
-   return operator_strs[operation];
+   return operator_string(this->operation);
 }
 
 ir_expression_operation