[OpenCL, test] Fix use of undef FileCheck var

[thopre]

Summary

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.

[yaxunl]

LGTM. Thanks.

clang/test/CodeGenOpenCL/fpmath.cl

 // RUN: %clang_cc1 %s -emit-llvm -o - -triple spir-unknown-unknown | FileCheck --check-prefix=CHECK --check-prefix=NODIVOPT %s
 // RUN: %clang_cc1 %s -emit-llvm -o - -triple spir-unknown-unknown -cl-fp32-correctly-rounded-divide-sqrt | FileCheck --check-prefix=CHECK --check-prefix=DIVOPT %s
 // RUN: %clang_cc1 %s -emit-llvm -o - -DNOFP64 -cl-std=CL1.2 -triple r600-unknown-unknown -target-cpu r600 -pedantic | FileCheck --check-prefix=CHECK-FLT %s
 // RUN: %clang_cc1 %s -emit-llvm -o - -DFP64 -cl-std=CL1.2 -triple spir-unknown-unknown -pedantic | FileCheck --check-prefix=CHECK-DBL %s
 
 typedef __attribute__(( ext_vector_type(4) )) float float4;
 
 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;
 }
 
 #if __OPENCL_C_VERSION__ >=120
 void printf(constant char* fmt, ...);
 
 void testdbllit(long *val) {
   // CHECK-FLT: float 2.000000e+01