GeezerVison

The cold exits use quite a bit of memory on JIT builds, and complicate memory allocation improvements (since they are the only immortal, shared-by-all-interpreters, unlinked, un-invalidatable executors). They also introduce a very subtle back-and-forth dance between executors when warming up side exits, and represent a significant proportion of the uops executed.

This:

• Removes them and puts all of their logic into the _EXIT_TRACE instruction (previously this logic was shared between the exit_to_trace label (only used by _EXIT_TRACE) and _COLD_EXIT...
• ...and also changes _EXIT_TRACE to use an oparg instead of an exit_index...
• ...which allows us to simplify the tier two instruction format.

Separately, I added some missing stats to _DYNAMIC_EXIT.

This results in a 34.7% reduction in traces executed, and a 1.6% reduction in uops executed (both corresponding to 3.7 billion fewer _COLD_EXITS). Per platform:

• aarch64-apple-darwin: 3% faster, 9% less memory
• aarch64-unknown-linux-gnu: 9% faster, 2% less memory
• i686-pc-windows-msvc: 3% faster
• x86_64-pc-windows-msvc: 3% faster
• x86_64-unknown-linux-gnu: 1% faster, 2% less memory

• Issue: Compiling tiny traces wastes lots of memory #116017