2d Arm Neon sdot op, and lowering to the intrinsic.

This adds Sdot2d op, which is similar to the usual Neon
intrinsic except that it takes 2d vector operands, reflecting the
structure of the arithmetic that it's performing: 4 separate
4-dimensional dot products, whence the vector<4x4xi8> shape.

This also adds a new pass, arm-neon-2d-to-intr, lowering
this new 2d op to the 1d intrinsic.

Reviewed By: nicolasvasilache

Differential Revision: https://reviews.llvm.org/D102504