erb.rb: Skip creating regexp

if stags and etags are not changed from default.

:putobject insn (of regexp) will be used instead of :toregexp insn.
This means that the regexp won't be compiled for every
`SimpleScanner#scan` call.

It may not be a good idea to apply this kind of optimization for all cases.
But I applied this because it is default scanner and used frequently and has
relatively large impact for benchmark like this:

* Before
app_erb 1.023

* After
app_erb 0.781

This commit fixes only the bottleneck of performance regression introduced
in r53412. For maintainability, I won't fix other small regressions like
additional overhead of method calls.

[ruby-core:73820] [Bug #12074]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@58842 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

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