Negated float comparison is wrong for NaN: !(x < 0.0) compiles to x >= 0.0 #9026

Negating a float comparison flips the operator, which is wrong for NaN:

let k = (x: float) => !(x < 0.0)

compiles to

function k(x) {
  return x >= 0.0;
}

k(NaN) returns false, but !(NaN < 0.0) is true.

The cause is Js_exp_make.not (compiler/core/js_exp_make.ml), which rewrites < to >=, >= to <, <= to > and > to <=. These are only complements when neither operand is NaN. The same function is used whenever the compiler negates a condition, e.g. when Js_stmt_make.if_ swaps the branches of an if with an empty then-branch, so other float comparisons can be affected too.

The rewrite is safe when the operands can't be NaN, e.g. for int, string and bigint comparisons. For floats, !(a < b) has to stay as it is.

Found by Codex while reviewing #9024.