cleanup irtype comments
remove R_FLAG and reassoc_running flag

The reassoc running flag wasn't necessary in new reassoc versions
anyway.
remove non-working auto_create_sync setting
rework alias analysis options, remove global on/off switch in

The global switch is not necessary as the specific switches can express
the same options.
cleanup flag code

Settle on 1 flag type and no different I_FLAG and E_FLAG
Add loop attribute to Phi node

This makes our usage of PhiMs to represent the observable behaviour of
non-terminating loops more obvious.
Having an explicit loop Phi constructor also fixes problems with Phi
getting optimized before the keep edge was added.