Optimize reference tracking and eliminate branching during returns and yields

[markshannon]

There are a few optimizations we can make to returns and yields, but they are somewhat related so I'm grouping them into a single issue.

During a return, the VM needs to make the returned reference "heap safe" (convert any borrowed references to strong references) and then pop and clear the frame, which can involve quite a lot of decrefs.
During a yield, the VM needs to make the returned reference heap safe and then pop the frame, but not clear it.

We want to minimize the amount of refcounting operations that we do.

Before we can do much else, we should split RETURN_VALUE and YIELD_VALUE into micro-ops:

macro(RETURN_VALUE) = _MAKE_HEAP_SAFE + _RETURN_VALUE

macro(YIELD_VALUE) = _MAKE_HEAP_SAFE + _YIELD_VALUE

so that we can optimize the uops independently

Eliminate making heap safe if we know that reference already is

If TOS is a strong reference _MAKE_HEAP_SAFE -> _NOP

Avoid the branch in _RETURN_VALUE by splitting into _RETURN_VALUE_GEN and _RETURN_VALUE_FUNC

To avoid using up opcodes, we'll need to do this in the JIT, not the interpreter.

Track borrows/immortals to optimize clearing frames

E.g. if a frame has four local variables, a, b, c, d but we can tell that a and d are immortal, or borrowed, then instead of looping over all the variables, we could emit code just to decref b and c.
This might end up bloating the code, as we would need to inline the decrefs, but it could be quite a lot faster.

Linked PRs

• gh-144540: Add _MAKE_HEAP_SAFE uop to eliminate unnecessary refcount operations in RETURN_VALUE and YIELD_VALUE #144414