[InstCombine] add even more tests for unsigned saturated add; NFC

The pattern-matching from rL354221 / rL354224 doesn't cover
these, so we're up to 8 different commuted possibilities.

There may still be 1 more variant of this pattern.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@354236 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed