Previously we'd check `x` for -0 by testing `(1.0 / x) == -Infinity`,
but this will yield the wrong results when `x` is a subnormal, i.e.
really close to 0.

In CSA we already perform bit checks to test for -0, so teach TurboFan
to do the same for comparisons to -0 (via `Object.is`). We introduce a
new NumberIsMinusZero simplified operator to handle the case where
SimplifiedLowering already knows that the input is a number.

Bug: chromium:903043, v8:6882
Change-Id: I0cb7c568029b461a92fc183104d5f359b4bfe7f4
Reviewed-on: https://chromium-review.googlesource.com/c/1328802
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57382}