fpu: handle raising Invalid for infzero in pick_nan_muladd

fpu: handle raising Invalid for infzero in pick_nan_muladd

For IEEE fused multiply-add, the (0 * inf) + NaN case should raise
Invalid for the multiplication of 0 by infinity.  Currently we handle
this in the per-architecture ifdef ladder in pickNaNMulAdd().
However, since this isn't really architecture specific we can hoist
it up to the generic code.

For the cases where the infzero test in pickNaNMulAdd was
returning 2, we can delete the check entirely and allow the
code to fall into the normal pick-a-NaN handling, because this
will return 2 anyway (input 'c' being the only NaN in this case).
For the cases where infzero was returning 3 to indicate "return
the default NaN", we must retain that "return 3".

For Arm, this looks like it might be a behaviour change because we
used to set float_flag_invalid | float_flag_invalid_imz only if C is
a quiet NaN.  However, it is not, because Arm target code never looks
at float_flag_invalid_imz, and for the (0 * inf) + SNaN case we
already raised float_flag_invalid via the "abc_mask &
float_cmask_snan" check in pick_nan_muladd.

For any target architecture using the "default implementation" at the
bottom of the ifdef, this is a behaviour change but will be fixing a
bug (where we failed to raise the Invalid exception for (0 * inf +
QNaN).  The architectures using the default case are:
 * hppa
 * i386
 * sh4
 * tricore

The x86, Tricore and SH4 CPU architecture manuals are clear that this
should have raised Invalid; HPPA is a bit vaguer but still seems
clear enough.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20241202131347.498124-2-peter.maydell@linaro.org