Shouldn't your snippet be using lzcnt? I can't see how this would result in the desired lookup.

for Zen5 rustc creates the following:

  utf8_sequence_length_lookup:
    shl edi, 24
    mov ecx, 274945
    not edi
    lzcnt eax, edi
    shl al, 2
    shrx rax, rcx, rax
    and al, 7
    ret

https://rust.godbolt.org/z/hz1eKjnaG