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Fixes the following minor issues:
* MSVC silently accepted runtime version of clamped negate in constexpr
  (a correctness issue that also generated marginally slower code).
* Checked negation did not use optimized builtins when available.

Change-Id: Ifd9a756d6620917a915ebdd583909824f2f86410
Reviewed-on: https://chromium-review.googlesource.com/573432
Commit-Queue: Justin Schuh <jschuh@chromium.org>
Reviewed-by: Erik Chen <erikchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#487267}