target/hppa: Fix 64-bit SHRPD instruction

When shifting the two joined 64-bit registers right, shift the upper
64-bit register to the left and the lower 64-bit register to the right
before merging them with OR.

Signed-off-by: Helge Deller <deller@gmx.de>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>