This extends the typing rule for NumberTrunc to deal with general number
inputs properly, thus addressing a long-standing TODO. We also add test
cases to ensure that the typing rule gets the corner cases for NaN and
-0 right.

Bug: v8:5267, v8:7109
Change-Id: Iedc541a0f4619f37da37ea36940f92472034cdf2
Reviewed-on: https://chromium-review.googlesource.com/792932Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49652}