i#1551 port to ARM: fix decode immediate bugs
i#1551 port to ARM: fix decode immediate bugs

Fixes several bugs:
+ The Thumb "modified immediates" must be OPSZ_4, not their native OPSZ_12b,
  due to tiling/shifting/rotating.
+ Negative immeds need to be created as opnd_create_immed_int(), not
 opnd_create_immed_uint().
+ The OP_blx immediates are signed, not unsigned.

Review-URL: https://codereview.appspot.com/197270043

AmesianX/dynamorio,sigma-random/dynamorio,bl4ckic3/dynamorio,daksunt/dynamorio,code4bones/dynamorio,daksunt/dynamorio,bl4ckic3/dynamorio,sigma-random/dynamorio,AmesianX/dynamorio,code4bones/dynamorio,daksunt/dynamorio,bl4ckic3/dynamorio,AmesianX/dynamorio,bl4ckic3/dynamorio,sigma-random/dynamorio,daksunt/dynamorio,code4bones/dynamorio,bl4ckic3/dynamorio,daksunt/dynamorio,AmesianX/dynamorio,code4bones/dynamorio,AmesianX/dynamorio,sigma-random/dynamorio,code4bones/dynamorio,sigma-random/dynamorio