[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

Author: Thomas Preud'homme

clang/test/CodeGenOpenCL/fpmath.cl

@@ -9,15 +9,15 @@ float spscalardiv(float a, float b) {
   // CHECK: @spscalardiv
   // CHECK: fdiv{{.*}},
   // NODIVOPT: !fpmath ![[MD:[0-9]+]]
-  // DIVOPT-NOT: !fpmath ![[MD:[0-9]+]]
+  // DIVOPT-NOT: !fpmath !{{[0-9]+}}
   return a / b;
 }
 
 float4 spvectordiv(float4 a, float4 b) {
   // CHECK: @spvectordiv
   // CHECK: fdiv{{.*}},
   // NODIVOPT: !fpmath ![[MD]]
-  // DIVOPT-NOT: !fpmath ![[MD]]
+  // DIVOPT-NOT: !fpmath !{{[0-9]+}}
   return a / b;
 }