Fix test check label.

Summary:
I noticed when manually modifying this test that it was passing when I
expected it to fail. Looks like the combination of LABEL and NOT on the
check does not work. This can also be seen when running FileCheck with
only that one -check-prefix (removing the additional -check-prefix=B):

/usr/local/google/home/tejohnson/llvm/llvm_11_build/./bin/llvm-link -S -internalize -only-needed /usr/local/google/home/tejohnson/llvm/llvm_11_build/test/Linker/Output/link-flags.ll.tmp.b.bc /usr/local/google/home/tejohnson/llvm/llvm_11_build/test/Linker/Output/link-flags.ll.tmp.c.bc | /usr/local/google/home/tejohnson/llvm/llvm_11_build/./bin/FileCheck /usr/local/google/home/tejohnson/llvm/llvm_11/test/Linker/link-flags.ll -check-prefix=CN
error: no check strings found with prefix 'CN:'

The CN prefix checks don't in fact need "LABEL" so remove that.

Reviewers: tra

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D14173

llvm-svn: 251655