[OpenCL, test] Fix use of undef FileCheck var

Clang test CodeGenOpenCL/fpmath.cl uses a variable defined in an earlier
CHECK-NOT directive. However, by definition the pattern in that
directive is not supposed to occur so no variable will be defined. This
commit solves the issue by using a regex match with the same regex as in
the definition. It also changes the definition into a regex match since
no variable is going to be defined.

Reviewed By: yaxunl

Differential Revision: https://reviews.llvm.org/D99857