Report instructions causing side-exits in `--ujit-stats`

[maximecb]

It would be very useful to know which instructions trigger exits to the interpreter and how often, and display a list sorted by decreasing frequency. That will help us target our efforts to increase coverage.

The simplest way to implement this would be to call a method which takes note of which opcode is at the exit_pc in ujit_gen_exit(). Maybe we can generate a call to a function that takes the exit PC as a parameter when RUBY_DEBUG is set.

https://github.com/Shopify/ruby/blob/microjit/ujit_codegen.c#L55