target/ppc: use locally stored msr and avoid indirect access

target/ppc: use locally stored msr and avoid indirect access

hreg_compute_hflags_value already stores msr locally to be used in most
of the logic in the routine however some instances are still using
env->msr which is unnecessary. Use locally stored value as available.

Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: BALATON Zoltan <balaton@eik.bme.hu>
Signed-off-by: Harsh Prateek Bora <harshpb@linux.ibm.com>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>