target/arm: Rearrange Floating-point data-processing (2 regs)

target/arm: Rearrange Floating-point data-processing (2 regs)

There are lots of special cases within these insns.  Split the
major argument decode/loading/saving into no_output (compares),
rd_is_dp, and rm_is_dp.

We still need to special case argument load for compare (rd as
input, rm as zero) and vcvt fixed (rd as input+output), but lots
of special cases do disappear.

Now that we have a full switch at the beginning, hoist the ISA
checks from the code generation.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20190215192302.27855-4-richard.henderson@linaro.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>