pypy-style constant promotion to pool

Proposal:

We should have a JIT constant pool and a JIT directive in the optimizer_bytecodes.c to promote certain values to constants. This would unlock significantly more future optimizations.

Steps needed:

1. Add a constant pool to the executor object, so we can keep promoted constant alive. We already need this to unlock more constant propagation opportunities. The pool would be a PyListObject living in the vmdata field here https://github.com/python/cpython/blob/main/Include/internal/pycore_optimizer.h#L43 .
2. Introduce a new optimizer macro: PROMOTE_TO_CONST_POOL and associated function promote_to_const_pool(ctx, obj) in optimizer_symbols.c. This macro will store any value it receives as a constant in the constant pool. This allows us to promote contants we see to the constant pool. This takes inspiration from RPython's hint(obj, promote=True)

What optimizations does this unlock?

1. More constant propagation: we can now constant propagate not just immortal stuff but non-immortal things as well.
2. Refcount removal. Anything in the the pool can use the cheaper _LOAD_CONST_INLINE_BORROW . Which will automatically benefit this work https://github.com/python/cpython/issues/134584 and vice versa. Ie, we automatically make it cheaper to load and store these values!
3. On GIL/FT, we can promote method objects to the pool as well, which will allow us to perform more optimizations on them, such as LOAD_ATTR_METHOD folding.

We should also perhaps explore promoting globals to pool constants. Though that's a little risky, since we don't want to hold large objects around longer than needed.

The first PR should implement the first two steps and optimization 1. Optimization 2 should be a separate PR.

Linked PRs

• gh-140968

Start with Include/internal/pycore_optimizer.h and the executor's vmdata field, then inspect optimizer_bytecodes.c and optimizer_symbols.c. The first PR is complete when the executor has a PyListObject constant pool, PROMOTE_TO_CONST_POOL and promote_to_const_pool exist, and non-immortal values receive constant propagation; refcount removal is explicitly separate.