This makes easier to debug scripts related to r59950.

* before

$ ./ruby --dump=insns -e '"#{a}"'
== disasm: #<ISeq:<main>@-e>============================================
0000 putobject        ""                                              (   1)[Li]
0002 putself
0003 opt_send_without_block <callinfo!mid:a, argc:0, FCALL|VCALL|ARGS_SIMPLE>, <callcache>
0006 dup
0007 branchiftype     5, 15
0010 dup
0011 opt_send_without_block <callinfo!mid:to_s, argc:0, FCALL|ARGS_SIMPLE>, <callcache>
0014 tostring
0015 concatstrings    2
0017 leave

* after

$ ./ruby --dump=insns -e '"#{a}"'
== disasm: #<ISeq:<main>@-e>============================================
0000 putobject        ""                                              (   1)[Li]
0002 putself
0003 opt_send_without_block <callinfo!mid:a, argc:0, FCALL|VCALL|ARGS_SIMPLE>, <callcache>
0006 dup
0007 branchiftype     T_STRING, 15
0010 dup
0011 opt_send_without_block <callinfo!mid:to_s, argc:0, FCALL|ARGS_SIMPLE>, <callcache>
0014 tostring
0015 concatstrings    2
0017 leave

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@61217 b2dd03c8-39d4-4d8f-98ff-823fe69b080e