ZJIT: Emit run-time check for write barrier

[tekknolagi]

Right now if we don't know statically that an object being written to the frame/an object is an immediate, we always emit a write barrier. Instead, we should emit a run-time check testing if it is false or an immediate and only conditionally fire the write barrier.

A reasonable first PR would guard if VM_ENV_FLAG_WB_REQUIRED is set before a setlocal and side-exit if so (thereby completely omitting the write barrier in the majority of cases)

[tekknolagi]

Linked PR crashes and I'm not sure why. We should be able to skip function calls in LIR that don't return anything (no need for block params).

[tekknolagi]

It would also be good to know when VM_ENV_FLAG_WB_REQUIRED is set and if we can somehow skip explicitly checking that.

[jacob-shops]

Working on this issue here