Add a separate ARM instruction format for Saturate instructions.

Add a separate ARM instruction format for Saturate instructions.
(I discovered 2 more copies of the ARM instruction format list, bringing the
total to 4!!  Two of them were already out of sync.  I haven't yet gotten into
the disassembler enough to know the best way to fix this, but something needs
to be done.)  Add support for encoding these instructions.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@110754 91177308-0d34-0410-b5e6-96231b3b80d8