For numbers, `JSGraph::Constant(Handle<Object> value)` first checks the
type of `value`, and then calls `JSGraph::Constant` for its type, which
in turn performs some further checks before returning a value.

This patch saves a few `if` checks by making the following changes:

- JSGraph::Constant(0) → JSGraph::ZeroConstant()
- JSGraph::Constant(1) → JSGraph::OneConstant()

Change-Id: I6946c280437b7f1fe80d7c45c6fd9b3571907e07
Reviewed-on: https://chromium-review.googlesource.com/840982Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Mathias Bynens <mathias@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50288}