target/ppc: Retain hflags_nmsr only for migration

target/ppc: Retain hflags_nmsr only for migration

We have eliminated all normal uses of hflags_nmsr.  We need
not even compute it except when we want to migrate.  Rename
the field to emphasize this.

Remove the fixme comment for migrating access_type.  This value
is only ever used with the current executing instruction, and
is never live when the cpu is halted for migration.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20210315184615.1985590-6-richard.henderson@linaro.org>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>