swift234

simd overlay: Use LLVM vector types.

This brings the David Owens benchmark from http://owensd.io/2015/06/27/performance-xcode7-beta-2.html from parity with simd.h-based C to 3x faster.

Before:

RenderGradient ([UInt32].withUnsafeMutablePointer (SIMD))                 │ 7.035851 │ 6.304739 │ 9.815832 │  1.212 │

After:

RenderGradient ([UInt32].withUnsafeMutablePointer (SIMD))                 │ 2.318357 │ 2.223325 │ 2.697981 │ 0.1490 │

This also addresses rdar://problem/21574425, since Builtin.add_VecNxIntM isn't overflow-checked, and overflow checks really aren't wanted when working with vector types directly.

Reapplying now that Nadav's fixed the ARM64 SelectionDAG issue this exposed before, and Arnold's fixed
yet another SelectionDAG issue exposed after that.

Swift SVN r30006