compile.c: optimize unnecessary concatstrings

* compile.c (iseq_peephole_optimize): optimize away unnecessary
  concatenation of single string, following tostring which always
  puts a String instance.
  https://github.com/ruby/ruby/pull/1626#discussion_r139285653

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@59945 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

compile.c

@@ -2509,6 +2509,14 @@ iseq_peephole_optimize(rb_iseq_t *iseq, LINK_ELEMENT *list, const int do_tailcal
 	}
     }
 
+    if (IS_INSN_ID(iobj, tostring)) {
+	LINK_ELEMENT *next = iobj->link.next;
+	if (IS_INSN(next) && IS_INSN_ID(next, concatstrings) &&
+	    OPERAND_AT(next, 0) == INT2FIX(1)) {
+	    REMOVE_ELEM(next);
+	}
+    }
+
     if (do_tailcallopt &&
 	(IS_INSN_ID(iobj, send) ||
 	 IS_INSN_ID(iobj, opt_aref_with) ||