Implements upcoming changes to @@search according to
https://github.com/tc39/ecma262/pull/627.

This also adds SameValue to CodeStubAssembler and extracts a part of
CSA::TruncateTaggedToFloat64.

BUG=v8:5339

Review-Url: https://codereview.chromium.org/2438683005
Cr-Commit-Position: refs/heads/master@{#41000}