[X86] Lowering addus/subus intrinsics to native IR

Summary: This revision improves previous version (rL330322) which has been reverted due to crashes.

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.

Reviewers: craig.topper, spatel, RKSimon

Reviewed By: craig.topper

Subscribers: mike.dvoretsky, DavidKreitzer, sroland, llvm-commits

Differential Revision: https://reviews.llvm.org/D46179

llvm-svn: 339650