Convert the remaining CheckedNumeric and ClampedNumeric methods to be
fully C++14 constexpr compliant. This also includes the following
dependent changes:
* The constexpr allowed some compile-time optimizations in ClampedAddOp
  and ClampedSubOp, which required restructuring the overflow handling
  to get correct saturation semantics. (New tests cover this.)
* The Clamped*FastAsmOp implementations included redundant saturation
  code, which was removed.

Bug: 612995
Change-Id: I0e0397968712ee06f8beff2f74c638cab01f34bb
Reviewed-on: https://chromium-review.googlesource.com/603127


Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Justin Schuh <jschuh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#505494}