X86 Tests: Unsigned saturation subtraction tests. NFC.

Summary:
Adding tests for D37534.

Commit on behalf of julia.koval@intel.com

Reviewers: n.bozhenov, zvi, spatel, DavidKreitzer

Reviewed By: zvi

Differential Revision: https://reviews.llvm.org/D37510

llvm-svn: 314305