[X86] Use UADDSAT/USUBSAT instead of ADDUS/SUBUS

Replace the X86ISD opcodes ADDUS and SUBUS with generic ISD opcodes
UADDSAT and USUBSAT. As a side-effect, this also makes codegen for
the @llvm.uadd.sat and @llvm.usub.sat intrinsics reasonable.

This only replaces use in the X86 backend, and does not move any of
the ADDUS/SUBUS X86 specific combines into generic codegen.

Differential Revision: https://reviews.llvm.org/D55787

llvm-svn: 349481