After this change, the functionality of the CodeStubAssembler should be
sufficient to generate non-trivial stubs (e.g. the KeyedLoadIC) with control
flow, variables and probing of internal meta data structures.

Specifically this patch:

* introduces a Label class, which allows stubs to construct graphs that don't
  have linear control graphs.
* introduces a Variable class. Variables can be bound to Node* values at
  different points in a non-linear control flow graph. In conjunction with the
  Label machinery, the CodeStubAssembler ensures that Phi nodes are inserted at
  the "minimal" set of merge points.
* adds Tail calling support to other Stubs and to any arbitrary code whose
  interface can be described by a CallInterfaceDescriptor.
* provides new macros for accessing FixedArray elements that are optimized for
  use with Smi values.

Review URL: https://codereview.chromium.org/1649723002

Cr-Commit-Position: refs/heads/master@{#33664}