[InstCombine] Canonicalize const arg for saturating adds

If a saturating add intrinsic has one constant argument, make sure
it is on the RHS. This will simplify further transformations.

This change is part of https://reviews.llvm.org/D54534.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@347769 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed