[PATCH] Allow FMAs in safe math mode in some cases when one operand of the fmul is either exactly 0.0 or exactly 1.0.

[Stephen Lin]

> What is the behavior of FMA if the intermediate result overflows?
> E.g. what does 2 * __DBL_MAX__ - __DBL_MAX__ give as FMA?
>
> Joerg

I could be wrong but I don't think it even makes sense to ask about
the intermediate result overflowing; I think the operation is defined
as given whatever value would be obtained via an infinite-precision
real number calculation, rounded at the end to the nearest
representable double at the end, so there's no intermediate register
to overflow. Presumably the result would be __DBL_MAX__ in the case
you give.