Fix problems in "[OpenCL] Enabling the usage of CLK_NULL_QUEUE as compare operand."

Summary: Fixed warnings in commit: https://reviews.llvm.org/rL290171

Reviewers: djasper, Anastasia

Subscribers: yaxunl, cfe-commits, bader

Differential Revision: https://reviews.llvm.org/D27981

llvm-svn: 290431