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/2258073002
Cr-Commit-Position: refs/heads/master@{#38978}