softfloat: Handle default NaN mode after pickNaNMulAdd, not before

softfloat: Handle default NaN mode after pickNaNMulAdd, not before

It is implementation defined whether a multiply-add of
(0,inf,qnan) or (inf,0,qnan) raises InvalidaOperation or
not, so we let the target-specific pickNaNMulAdd function
handle this. This means that we must do the "return the
default NaN in default NaN mode" check after the call,
not before. Correct the ordering, and restore the comment
from the old propagateFloat64MulAddNaN() that warned about
this corner case.

This fixes a regression from 2.11 for Arm guests where we would
incorrectly fail to set the Invalid flag for these cases.

Cc: [email protected]
Signed-off-by: Peter Maydell <[email protected]>
Reviewed-by: Richard Henderson <[email protected]>
Reviewed-by: Alex Bennée <[email protected]>
Tested-by: Alex Bennée <[email protected]>
Message-id: 20180504100547[email protected]