JIT: Use any reaching def for unreachable uses in incremental SSA builder

[jakobbotsch]

Also give unreachable stores an SSA number.

Fix #109971

[jakobbotsch]

cc @dotnet/jit-contrib PTAL @EgorBo

There really is no correct reaching definition in this case, so any reaching definition will do here. This matches what happens normally in the JIT when various phases optimize control flow after SSA has been built without removing the now-unreachable blocks.

Diffs. They happen because we now create some more assertions for these uses which push out other assertions.

[EgorBo]

LGTM

They happen because we now create some more assertions for these uses which push out other assertions.

I wonder if we should only accept the most profitable assertions if we already consumed 80% of the budget

[jakobbotsch]

I wonder if we should only accept the most profitable assertions if we already consumed 80% of the budget

Sounds like a good idea.. Or we should look into why we cannot just allow more assertions through.