This CL brings unary op assembler structure closer to that of binary
ops assemblers:

- Decrement, Increment, Negate call into UnaryOpWithFeedback,
- which takes lambdas specifying smi, float, and bigint logic.
- BitwiseNot is different in that it still dispatches using
  TaggedToWOrd32OrBigIntWithFeedback.
- These methods are all implemented in the (hidden)
  UnaryOpAssemblerImpl class.
- The header only exposes UnaryOpAssembler with the bare minimum of
  API.

The last point is the remaining major divergence from binary op
assemblers. I just like how this avoids useless implementation details
in the header.

Bug: v8:8888
Change-Id: I0ac4695483950356885301234d58c1900904aa92
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2214830
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68004}