[HIP-Clang, test] Fix use of undef FileCheck var

Commit 81295213 changed a line defining
PREFIX in clang test CodeGenCUDA/device-stub.cu into a CHECK-NOT
directive. All following lines using PREFIX are therefore using an
undefined variable since the pattern defining PREFIX is not supposed to
occur and CHECK-NOT are checked independently.

This commit replaces all uses of PREFIX by the regex used to define it,
thereby avoiding the problem.

Reviewed By: yaxunl

Differential Revision: https://reviews.llvm.org/D99831