target-arm: fix neon vshrn/vrshrn ops

In the existing code shift value is clobbered during the pass loop.
This patch changes the code so that it stores the intermediate
result in the target neon register directly and eliminates the need
to use a temporary to hold the intermediate value thus leaving the
shift value in the temporary variable intact. This is a new patch
in this version of the patch series.

Signed-off-by: Juha Riihimäki <juha.riihimaki@nokia.com>
Acked-by: Laurent Desnogues <laurent.desnogues@gmail.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
