target/ppc: Refactor VSX_MAX_MINC helper

Refactor xs{max,min}cdp VSX_MAX_MINC helper to prepare for
xs{max,min}cqp implementation.

Signed-off-by: Víctor Colombo <victor.colombo@eldorado.org.br>
Signed-off-by: Matheus Ferst <matheus.ferst@eldorado.org.br>
Message-Id: <20220225210936.1749575-45-matheus.ferst@eldorado.org.br>
Signed-off-by: Cédric Le Goater <clg@kaod.org>