[FPEnv] Remove inaccurate comments regarding signaling NaN for isless

By draft of C23 (https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2912.pdf),
the description for isless macro under 7.12.17.3 says,

The isless macro determines whether its first argument is less than its second
argument. The value of isless(x,y) is always equal to (x)< (y); however, unlike
(x) < (y), isless(x,y) does not raise the invalid floating-point exception when
x and y are unordered and neither is a signaling NaN.

isless should trap when encountering signaling NaN.

Reviewed By: jcranmer-intel, efriedma

Differential Revision: https://reviews.llvm.org/D134407