[RFC] FileCheck: (dis)allowing unused prefixes

I'm not very familiar with FileCheck yet so this may be a silly question. I noticed that TableGen/GlobalISelEmitter-immAllZeroOne.td is on the list, so I checked it out. If I understand correctly, I expect GISEL-OPT and/or GISEL-NOOPT to be misspelled somewhere, but they are not. Can someone sort me out?

// RUN: llvm-tblgen -gen-global-isel -warn-on-skipped-patterns -optimize-match-table=false -I %p/../../include -I %p/Common %s -o - | FileCheck -check-prefixes=GISEL-NOOPT,GISEL %s
// RUN: llvm-tblgen -gen-global-isel -warn-on-skipped-patterns -optimize-match-table=true -I %p/../../include -I %p/Common %s -o - | FileCheck -check-prefixes=GISEL-OPT,GISEL %s