target/ppc: Refactor VSX_SCALAR_CMP_DP

Refactor VSX_SCALAR_CMP_DP, changing its name to VSX_SCALAR_CMP and
prepare the helper to be used for quadword comparisons.

Suggested-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Víctor Colombo <victor.colombo@eldorado.org.br>
Signed-off-by: Matheus Ferst <matheus.ferst@eldorado.org.br>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220225210936.1749575-41-matheus.ferst@eldorado.org.br>
Signed-off-by: Cédric Le Goater <clg@kaod.org>