YJIT: Specialize String#dup

[k0kubun]

This PR skips pushing a frame for String#dup when a receiver is a rb_cString and doesn't have instance variables.

It speeds up mail by 0.5%.

before: ruby 3.4.0dev (2024-11-14T17:29:47Z master 4074c6b427) +YJIT +PRISM [x86_64-linux]
after: ruby 3.4.0dev (2024-11-14T21:35:54Z yjit-str-dup e4e526ed38) +YJIT +PRISM [x86_64-linux]

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bench  before (ms)  stddev (%)  after (ms)  stddev (%)  after 1st itr  before/after
mail   69.8         0.4         69.4        0.2         1.004          1.005
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