[ARM] Some saturation instructions not DSP-only

Summary:
Commit 276701 requires that targets have the DSP extensions to use
certain saturating instructions. This requires some corrections.

For ARM ISA the instructions in question are available in all v6*
architectures.

For Thumb2, the instructions in question are available from v6T2.
SSAT and USAT are part of the base architecture while SSAT16 and
USAT16 require the DSP extensions.

Reviewers: rengolin

Subscribers: aemerson, rengolin, samparker, llvm-commits

Differential Revision: https://reviews.llvm.org/D23010

llvm-svn: 277439