Do not emit shape transition warnings when YJIT is compiling

[Bug #20522]

If `Warning.warn` is redefined in Ruby, emitting a warning would invoke
Ruby code, which can't safely be done when YJIT is compiling.

Co-authored-by: Jean Boussier <jean.boussier@gmail.com>
Co-authored-by: Takashi Kokubun <takashikkbn@gmail.com>