LGTM with one more consistency tweak.
Thanks for the patch. Sorry, I think another pass is needed here for consistency, the messages look like so (no quotes, single sentence):
test/MC/AArch64/invalid-instructions-spellcheck.s 28:// CHECK: error: unrecognized instruction mnemonic, did you mean: add, addp, adds, addv, fadd, madd? 35:// CHECK-NO-NEON: error: unrecognized instruction mnemonic, did you mean: add, adds, fadd, madd?