target/arm: Expand frint_fns for MO_8

Simplify indexing of this array.  This will allow folding
of the illegal esz == 0 into the normal fn == NULL check.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20220527181907.189259-93-richard.henderson@linaro.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>