ARM: use vqdmull and vqadds/vqsubs to implement vqdmlal/vqdmlsl

The NEON intrinsics vqdmlal and vqdmlsl are really just combinations of a
saturating-doubling-multiply (vqdmull) and a saturating add/sub, so now that
LLVM can spot those patterns Clang should emit them instead of specialised
intrinsics.

Feature already tested by existing ARM NEON intrinsics tests.

llvm-svn: 189462