Revert "[OpenCL] Enabling the usage of CLK_NULL_QUEUE as compare operand."

This reverts commit r290171. It triggers a bunch of warnings, because
the new enumerator isn't handled in all switches. We want a warning-free
build.

Replied on the commit with more details.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@290173 91177308-0d34-0410-b5e6-96231b3b80d8