[ARM] Add qadd lowering from a sadd_sat

[ARM] Add qadd lowering from a sadd_sat

This lowers a sadd_sat to a qadd by treating it as legal. Also adds qsub at the
same time.

The qadd instruction sets the q flag, but we already have many cases where we
do not model this in llvm.

Differential Revision: https://reviews.llvm.org/D68976

llvm-svn: 375411