compile.c: apply opt_str_freeze to String#-@ (uminus)

The same optimization used for "literal string".freeze
can easily apply to uminus without introducing any
compatibility problems.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@57828 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

bootstraptest/test_insns.rb

@@ -178,6 +178,7 @@ tests = [
   },
 
   [ 'opt_str_freeze', %q{ 'true'.freeze }, ],
+  [ 'opt_str_freeze', %q{ -'true' }, ],
   [ 'opt_str_freeze', <<~'},', ], # {
     class String
       def freeze

compile.c

@@ -5188,7 +5188,8 @@ iseq_compile_each(rb_iseq_t *iseq, LINK_ANCHOR *const ret, NODE *node, int poppe
 	 *   "literal".freeze -> opt_str_freeze("literal")
 	 */
 	if (node->nd_recv && nd_type(node->nd_recv) == NODE_STR &&
-	    node->nd_mid == idFreeze && node->nd_args == NULL &&
+	    (node->nd_mid == idFreeze || node->nd_mid == idUMinus) &&
+	    node->nd_args == NULL &&
 	    ISEQ_COMPILE_DATA(iseq)->current_block == NULL &&
 	    ISEQ_COMPILE_DATA(iseq)->option->specialized_instruction) {
 	    VALUE str = rb_fstring(node->nd_recv->nd_lit);