[ARM] Lower (select_cc k k (select_cc ~k ~k x)) into (SSAT l_k x)

Summary:
SSAT saturates an integer, making sure that its value lies within
an interval [-k, k]. Since the constant is given to SSAT as the
number of bytes set to one, k + 1 must be a power of 2, otherwise
the optimization is not possible. Also, the select_cc must use <
and > respectively so that they define an interval.

Reviewers: mcrosier, jmolloy, rengolin

Subscribers: aemerson, rengolin, llvm-commits

Differential Revision: http://reviews.llvm.org/D21372

llvm-svn: 273581