ARM64: diagnose use of v16-v31 in certain indexed NEON instructions.

Someone couldn't bear to have a completely orthogonal set of floating-point
registers, so we've got some instructions that only accept v0-v15 (coming in
ARMv9, V128_prime: you're allowed v2, v3, v5, v7, ...).

Anyway, we were permitting even the out of range registers during assembly
(CodeGen handled it correctly). This adds a diagnostic.