target/arm: Refactor disas_simd_indexed size checks

The integer size check was already outside of the opcode switch;
move the floating-point size check outside as well.  Unify the
size vs index adjustment between fp and integer paths.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20180228193125.20577-4-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>