FileCheck expects directives like:

// COM:
// CHECK:
// CHECK-NEXT:
// CHECK-SAME:
// CHECK-EMPTY:
// CHECK-NOT:
// CHECK-COUNT:
// CHECK-DAG:
// CHECK-LABEL:

compiletest supports //@ revisions: a b c and passes the revision names as --check-prefixes=a,b,c to FileCheck. They look like

// a:
// b-NOT:
// c-SAME:

However, FileCheck has special default/reserved prefixes like CHECK or COM, and if the test writer uses those as revision names, it becomes really confusing: //@ revisions: CHECK COM. We should deny those.

Also we should deny NEXT, SAME, EMPTY, NOT, COUNT, DAG and LABEL, because

// next:
// same-SAME:
// not-NOT:

is equally confusing.