target-mips: Fix incorrect shift for SHILO and SHILOV

helper_shilo has not been shifting an accumulator value correctly for negative
values in 'shift' field. Minor optimization for shift=0 case.
This change also adds tests that will trigger issue and check for regressions.

Signed-off-by: Petar Jovanovic <petarj@mips.com>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Reviewed-by: Eric Johnson <ericj@mips.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
