Adds support for count operations to the interpreter. Deals with count
operations on locals, globals, context allocated variables and named and
keyed properties.

Adds the following bytecodes:
  ToNumber
  Inc
  Dec

BUG=v8:4280
LOG=N
TBR=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/1416623003

Cr-Commit-Position: refs/heads/master@{#31484}