Add CHECK-NOT for older target-cpu in test case

Author: amy-kwan

llvm/test/CodeGen/PowerPC/aix-cpu-version-multifunction.ll

@@ -7,6 +7,7 @@
 ; CHECK-NEXT: .csect ..text..[PR],5
 ; CHECK-NEXT: .rename ..text..[PR],""
 ; CHECK-NEXT: .machine "PWR10"
+; CHECK-NOT:  .machine "PWR8"
 
 source_filename = "file.c"