[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@375411 91177308-0d34-0410-b5e6-96231b3b80d8