target-arm: Simplify three-register pairwise code

Since we know that the case of (pairwise && q) has been caught
earlier, we can simplify the register setup code for each pass
in the three-register-same-size Neon loop.

Signed-off-by: Juha Riihimäki <juha.riihimaki@nokia.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
