YJIT: Fix ruby2_keywords splat+rest and drop bogus checks

YJIT didn't guard for ruby2_keywords hash in case of splat calls that
land in methods with a rest parameter, creating incorrect results.

The compile-time checks didn't correspond to any actual effects of
ruby2_keywords, so it was masking this bug and YJIT was needlessly
refusing to compile some code. About 16% of fallback reasons in
`lobsters` was due to the ISeq check.

We already handle the tagging part with
exit_if_supplying_kw_and_has_no_kw() and should now have a dynamic guard
for all splat cases.

Note for backporting: You also need 7f51959ff1.

[Bug #20195]

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