target/ppc: Disconnect hflags from MSR

target/ppc: Disconnect hflags from MSR

Copying flags directly from msr has drawbacks: (1) msr bits
mean different things per cpu, (2) msr has 64 bits on 64 cpus
while tb->flags has only 32 bits.

Create a enum to define these bits.  Document the origin of each bit
and validate those bits that must match MSR.  This fixes the
truncation of env->hflags to tb->flags, because we no longer
have hflags bits set above bit 31.

Most of the code in ppc_tr_init_disas_context is moved over to
hreg_compute_hflags.  Some of it is simple extractions from msr,
some requires examining other cpu flags.  Anything that is moved
becomes a simple extract from hflags in ppc_tr_init_disas_context.

Several existing bugs are left in ppc_tr_init_disas_context, where
additional changes are required -- to be addressed in future patches.

Remove a broken #if 0 block.

Reported-by: Ivan Warren <ivan@vmfacility.fr>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20210323184340.619757-3-richard.henderson@linaro.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>