s390x/tcg: Rework MMU selection for instruction fetches

s390x/tcg: Rework MMU selection for instruction fetches

Instructions are always fetched from primary address space, except when
in home address mode. Perform the selection directly in cpu_mmu_index().

get_mem_index() is only used to perform data access, instructions are
fetched via cpu_lduw_code(), which translates to cpu_mmu_index(env, true).

We don't care about restricting the access permissions of the TLB
entries anymore, as we no longer enter PRIMARY entries into the
SECONDARY MMU. Cleanup related code a bit.

Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Message-Id: <20190816084708.602-4-david@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>