target/arm: Restructure disas_fp_int_conv

target/arm: Restructure disas_fp_int_conv

For opcodes 0-5, move some if conditions into the structure
of a switch statement.  For opcodes 6 & 7, decode everything
at once with a second switch.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20190215192302.27855-2-richard.henderson@linaro.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>