This CL adds a constexpr int31 == operator that returns
a constexpr bool value.

It also fixes a bug that this introduces because branch generation
for ternary operators incorrectly assumed that the condition value
was bool, whereas it can also be constexpr bool now.

Change-Id: Ie7d3497c9d1112c121b4a29561fe065b5bf01fb6
Reviewed-on: https://chromium-review.googlesource.com/1106343Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Théotime Grohens <theotime@google.com>
Cr-Commit-Position: refs/heads/master@{#53867}