[ARM] Lower sadd_sat to qadd8 and qadd16

[ARM] Lower sadd_sat to qadd8 and qadd16

Lower the target independent signed saturating intrinsics to qadd8 and qadd16.
This custom lowers them from a sadd_sat, catching the node early before it is
promoted. It also adds a QADD8b and QADD16b node to mean the bottom "lane" of a
qadd8/qadd16, so that we can call demand bits on it to show that it does not
use the upper bits.

Also handles QSUB8 and QSUB16.

Differential Revision: https://reviews.llvm.org/D68974

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@375402 91177308-0d34-0410-b5e6-96231b3b80d8