target/ppc: fmadd check for excp independently

Current order of checking does not confirm with the spec
(ISA 3.0: MultiplyAddDP page-469). Change the order and make them
independent of each other.

For example: a = infinity, b = zero, c = SNaN, this should set both
VXIMZ and VXNAN

Signed-off-by: Nikunj A Dadhania <nikunj@linux.vnet.ibm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
