The `catch_except_p` flag is used for communicating between parent and
child iseq's that a throw instruction was emitted. So for example if a
child iseq has a throw in it and the parent wants to catch the throw, we
use this flag to communicate to the parent iseq that a throw instruction
was emitted.

This flag is only useful at compile time, it only impacts the
compilation process so it seems to be fine to move it from the iseq body
to the compile_data struct.

Co-authored-by: Aaron Patterson <tenderlove@ruby-lang.org>