The CheckSmi in String.fromCodePoint() is unnecessary and even leads to
unnecessary deoptimizations, since the CheckBounds already does the
right thing, plus it also handles HeapNumbers (in Signed32 range) and
properly identifies zeros.

Bug: v8:8238
Change-Id: I73bf7a70c3cd718c987f112ceb928188c0534cd5
Reviewed-on: https://chromium-review.googlesource.com/c/1262675Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56395}