[ARM] Improve sxta{b|h} and uxta{b|h} tests

Created a Thumb2 predicated pattern matcher that uses Thumb2 and
HasT2ExtractPack and used it to redefine the patterns for sxta{b|h}
and uxta{b|h}. Also used the similar patterns to fill in isel pattern
gaps for the corresponding instructions in the ARM backend.
The patch is mainly changes to tests since most of this functionality
appears not to have been tested.

Differential Revision: https://reviews.llvm.org/D23273

llvm-svn: 278207