gh-139757: Treat call specially in JIT assembly backend optimizer on x86-64 and AArch64

[Fidget-Spinner]

On x86-64 and AArch64, a call is effectively a jump to a label with some caveats. As such, the assembly flow optimizer must treat it as such, or it risks emitting invalid optimizations.

The bug triggers in the following code:

_JIT_ENTRY:
    call PyStackRef_CLOSE

PyStackRef_CLOSE:
    bar

The assembly optimizer currently doesn't see PyStackRef_CLOSE as being used, and just optimizes the whole thing away.

The fix informs the assembly optimizer we might jump to PyStackRef_CLOSE, thus the label is used and to preserve it.

I verified it fixes the crash on FT builds on my system.

The reason why we didn't see this earlier is that our stencils were at -O3 and that squashed them all into one function. After we switched to -Os, bigger stencils get outlined. It just so happens the biggest stencils are on FT.

• Issue: JIT assembly optimizer leaves some redundant branches #139757

[savannahostrowski]

LGTM - thanks for the back and forth on this!

[Fidget-Spinner]

I'm merging this as it's fixing an actual bug that causes malformed code in the JIT. We can refine it later if we want, but I have gone through a lot of discussion with Savannah about the correctness of this PR, and I feel confident on it.