target/ppc: Tidy helper_fadd, helper_fsub

target/ppc: Tidy helper_fadd, helper_fsub

Tidy the invalid exception checking so that we rely on softfloat for
initial argument validation, and select the kind of invalid operand
exception only when we know we must.  Pass and return float64 values
directly rather than bounce through the CPU_DoubleU union.

Note that because we know float_flag_invalid was set, we do not have
to re-check the signs of the infinities.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>