The Object.is builtin provides an entry point to the abstract operation
SameValue, which properly distinguishes -0 and 0, and also identifies
NaNs. Most of the time you don't need these, but rather just regular
strict equality, but when you do, Object.is(o, -0) is the most readable
way to check for minus zero.

This is for example used in Node.js by formatNumber to properly print -0
for negative zero. However since the builtin thus far implemented as C++
builtin and TurboFan didn't know anything about it, Node.js considering
to go with a more performant, less readable version (which also makes
assumptions about the input value) in

  https://github.com/nodejs/node/pull/15726

until the performance of Object.is will be on par (so hopefully we can
go back to Object.is in Node 9).

This CL ports the baseline implementation of Object.is to CSA, which
is pretty straight-forward since SameValue is already available in
CodeStubAssembler, and inlines a few interesting cases into TurboFan,
i.e. comparing same SSA node, and checking for -0 and NaN explicitly.

On the micro-benchmarks we go from

  testNumberIsMinusZero: 1000 ms.
  testObjectIsMinusZero: 929 ms.
  testObjectIsNaN: 954 ms.
  testObjectIsSame: 793 ms.
  testStrictEqualSame: 104 ms.

to

  testNumberIsMinusZero: 89 ms.
  testObjectIsMinusZero: 88 ms.
  testObjectIsNaN: 88 ms.
  testObjectIsSame: 86 ms.
  testStrictEqualSame: 105 ms.

which is a nice 10x to 11x improvement and brings Object.is on par with
strict equality for most cases.

Drive-by-fix: Also refactor and optimize the SameValue check in the
CodeStubAssembler to avoid code bloat (by not inlining StrictEqual
into every user of SameValue, and also avoiding useless checks).

Bug: v8:6882
Change-Id: Ibffd8c36511f219fcce0d89ed4e1073f5d6c6344
Reviewed-on: https://chromium-review.googlesource.com/700254Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48275}