i#1686 arm decoder testing: TYPE_I_b8_b0 bug fix
i#1686 arm decoder testing: TYPE_I_b8_b0 bug fix

Fixes a decoding error in i16x8_0 where TYPE_I_b8_b0 shifted the top 12
bits too far.  Found by testing on random inputs.

Review-URL: https://codereview.appspot.com/229010043

AmesianX/dynamorio,bl4ckic3/dynamorio,AmesianX/dynamorio,AmesianX/dynamorio,bl4ckic3/dynamorio,daksunt/dynamorio,code4bones/dynamorio,AmesianX/dynamorio,bl4ckic3/dynamorio,code4bones/dynamorio,code4bones/dynamorio,code4bones/dynamorio,daksunt/dynamorio,daksunt/dynamorio,bl4ckic3/dynamorio,code4bones/dynamorio,AmesianX/dynamorio,daksunt/dynamorio,daksunt/dynamorio,bl4ckic3/dynamorio