port 56429fc1 (r38978)

  original commit message:
  Introduced MachineType::TaggedSigned() and TaggedPointer().

  The idea is to quit using the representational dimension of Type, and
  instead encode this information in the MachineRepresentation (itself
  lightly wrapped in MachineType, along with MachineSemantic).

  There are three parts to the whole change:

  1) Places that set the machine representation - constant nodes, loads nad
     stores, global object and native context specialization.

  2) Places that propagate type/representation - this is representation
     inference (aka simplified lowering). At the end of this process we
     expect to have a MachineRepresentation for every node. An interesting
     part of this is phi merging.

  3) Places that examine representation - WriteBarrier elimination does this.
     Currently it's looking at the Type representation dimension, but as
     a part of this change (or in a soon-to-follow change) it can simply
     examine the MachineRepresentation.

BUG=

Review-Url: https://codereview.chromium.org/2293603003
Cr-Commit-Position: refs/heads/master@{#38993}