Fix test check label.

[tejohnson]

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.

[tra]

LGTM.

test/Linker/link-flags.ll

 ; RUN: llvm-as %S/Inputs/linkage.b.ll -o %t.b.bc
 ; RUN: llvm-as %S/Inputs/linkage.c.ll -o %t.c.bc
 ; RUN: llvm-link -S %t.b.bc %t.c.bc | FileCheck %s -check-prefix=B -check-prefix=C -check-prefix=CU
 ; RUN: llvm-link -S -only-needed %t.b.bc %t.c.bc | FileCheck %s -check-prefix=B -check-prefix=C -check-prefix=CN
 ; RUN: llvm-link -S -internalize %t.b.bc %t.c.bc | FileCheck %s -check-prefix=B -check-prefix=CI
 ; RUN: llvm-link -S -internalize -only-needed %t.b.bc %t.c.bc | FileCheck %s -check-prefix=B -check-prefix=CN
 
 C-LABEL: @X = global i32 5
 CI-LABEL: @X = internal global i32 5
 CU-LABEL:@U = global i32 6
 CI-LABEL:@U = internal global i32 6
-CN-LABEL-NOT:@U
+CN-NOT:@U
 
 B-LABEL: define void @bar() {
 
 C-LABEL: define i32 @foo()
 CI-LABEL: define internal i32 @foo()
 
 CU-LABEL:define i32 @unused() {
 CI-LABEL:define internal i32 @unused() {
-CN-LABEL-NOT:@unused()
+CN-NOT:@unused()