Refactor the ARM 'setend' instruction pattern. Use a single instruction pattern

Refactor the ARM 'setend' instruction pattern. Use a single instruction pattern
and handle the operand explicitly. Flesh out encoding information. Add an
explicit disassembler testcase for the instruction.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@116432 91177308-0d34-0410-b5e6-96231b3b80d8