The infrastructure for atomic operations in the rs6000 port has not
utilized the cleaner GCC internal APIs for manipulating memmodel.  The
appended patch converts the direct casts and use of memmodel_from_int
to consistently use memmodel_base.  This also allows the SYNC variants
to be removed from switch case statements.  This should not change any
semantics or behavior of atomics in PowerPC code generation.

Comments?

Bootstrapped on powerpc-ibm-aix7.1.0.0

        * config/rs6000/rs6000.c (rs6000_pre_atomic_barrier): Remove SYNC
        variants cases from switch.
        (rs6000_post_atomic_barrier): Same.
        (rs6000_expand_atomic_compare_and_swap): Use memmodel_base.
        (rs6000_expand_atomic_exchange): Same.
        (rs6000_expand_atomic_op): Same.
        * config/rs6000/sync.md (mem_thread_fence): Use memodel_base. Remove
        SYNC variants cases from switch.
        (atomic_load): Same.
        (atomic_store): Same.

Thanks, David