Add constant folding rules for floating-point comparison

This change handles all 6 regular comparison types in two variations,
ordered (true if values are ordered *and* comparison is true) and
unordered (true if values are unordered *or* comparison is true).

Ordered comparison matches the default floating-point behavior on host
but we use std::isnan to check ordering explicitly anyway.

This change also slightly reworks the floating-point folding support
code to make it possible to define a folding operation that returns
boolean instead of floating point.

These tests exhaustively test ordered/unordered comparisons for
float/double.

Since for NaN inputs the comparison result doesn't depend on the
comparison function, we just test == and !=; NaN inputs result in true
unordered comparisons and false ordered comparisons.