ARM assembly parsing and encoding for SSAT instruction.

Fix the Rn register encoding for both SSAT and USAT. Update the parsing of the
shift operand to correctly handle the allowed shift types and immediate ranges
and issue meaningful diagnostics when an illegal value or shift type is
specified. Add aliases to parse an ommitted shift operand (default value of
'lsl #0').

Add tests for diagnostics and proper encoding.

llvm-svn: 135990