target/ppc: Change VSX instructions behavior to fill with zeros

target/ppc: Change VSX instructions behavior to fill with zeros

ISA v3.1 changed some VSX instructions behavior by changing what the
other words/doubleword in the result should contain when the result is
only one word/doubleword. e.g. xsmaxdp operates on doubleword 0 and
saves the result also in doubleword 0.
Before, the second doubleword result was undefined according to the
ISA, but now it's stated that it should be zeroed.

Even tough the result was undefined before, hardware implementing these
instructions already filled these fields with 0s. Changing every ISA
version in QEMU to this behavior makes the results match what happens
in hardware.

Signed-off-by: Víctor Colombo <victor.colombo@eldorado.org.br>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220204181944.65063-1-victor.colombo@eldorado.org.br>
Signed-off-by: Cédric Le Goater <clg@kaod.org>