target/ppc: Optimise VSX_LOAD_SCALAR_DS and VSX_VECTOR_LOAD_STORE

target/ppc: Optimise VSX_LOAD_SCALAR_DS and VSX_VECTOR_LOAD_STORE

A few small optimisations:

In VSX_LOAD_SCALAR_DS() we can don't need to read the VSR via
get_cpu_vsrh().

Split VSX_VECTOR_LOAD_STORE() into two functions. Loads only need to
write the VSRs (set_cpu_vsr*()) and stores only need to read the VSRs
(get_cpu_vsr*())

Thanks to Mark Cave-Ayland for the suggestions.

Signed-off-by: Anton Blanchard <anton@ozlabs.org>
Message-Id: <20190509103545.4a7fa71a@kryten>
Reviewed-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>