R600/SI: Remove dead code and add missing tests.

This probably was killed by some generic DAGCombiner
improvements in checking the TargetBooleanContents instead
of just 1.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213471 91177308-0d34-0410-b5e6-96231b3b80d8