Lowering x86 adds/addus/subs/subus intrinsics (llvm part)

This is the patch that lowers x86 intrinsics to native IR
in order to enable optimizations. The patch also includes folding
of previously missing saturation patterns so that IR emits the same
machine instructions as the intrinsics.

Patch by tkrupa

Differential Revision: https://reviews.llvm.org/D44785

llvm-svn: 330322