fix optimization for hash aset/aref with fstring

	Patch by Eric Wong [ruby-core:78797].
	I don't like the idea of making insns.def any bigger to support
	a corner case, and "test_hash_aref_fstring_identity" shows
	how contrived this is.

	[ruby-core:78783] [Bug #12855]
	adjust indent [ci skip]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_4@57848 b2dd03c8-39d4-4d8f-98ff-823fe69b080e