Prior to this change, the exponentiation operator was rewritten by the
parser to a call of the Math.pow builtin. However, Math.pow does not
accept BigInt arguments, while the exponentiation operator must accept
them.

This CL
- removes the parser's special treatment of ** and **=, treating them
  like any other binary op instead.
- adds a TFC builtin Exponentiate that does the right thing for
  all inputs.
- adds interpreter bytecodes Exp and ExpSmi whose handlers call the
  Exponentiate builtin. For simplicity, they currently always collect
  kAny feedback.
- adds a Turbofan operator JSExponentiate with a typed-lowering to
  the existing NumberPow and a generic-lowering to the Exponentiate
  builtin. There is currently no speculative lowering.

Note that exponentiation for BigInts is actually not implemented yet,
so we can't yet test it.

Bug: v8:6791
Change-Id: Id90914c9c3fce310ce01e715c09eaa9f294f4f8a
Reviewed-on: https://chromium-review.googlesource.com/785694Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Mythri Alle <mythria@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49696}