target/ppc: Merge COMPUTE_CLASS and COMPUTE_FPRF

Instead of computing an artificial "class" bitmask then converting that
to the fprf value, compute the final value from the start.

Reorder the tests to check the most likely cases first.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Tested-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20230523202507.688859-1-richard.henderson@linaro.org>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>