YJIT: Specialize `String#[]` (`String#slice`) with fixnum arguments (#12069)

* YJIT: Specialize `String#[]` (`String#slice`) with fixnum arguments

String#[] is in the top few C calls of several YJIT benchmarks:
liquid-compile rubocop mail sudoku

This speeds up these benchmarks by 1-2%.

* YJIT: Try harder to get type info for `String#[]`

In the large generated code of the mail gem the context doesn't have
the type info.  In that case if we peek at the stack and add a guard
we can still apply the specialization
and it speeds up the mail benchmark by 5%.

Co-authored-by: Maxime Chevalier-Boisvert <maxime.chevalierboisvert@shopify.com>
Co-authored-by: Takashi Kokubun (k0kubun) <takashikkbn@gmail.com>

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Co-authored-by: Maxime Chevalier-Boisvert <maxime.chevalierboisvert@shopify.com>
Co-authored-by: Takashi Kokubun (k0kubun) <takashikkbn@gmail.com>

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