This CL introduces a concept of UseInfo (but internally it still
uses machine types). The idea of UseInfo is to separate the concept
of truncation (what information is actually used by the user node)
and the concept of preferred representation. At the moment, the
truncation is (clumsily) represented by the type part of the
underlying machine type (UseInfo::type_).

Moreover, in this CL, we never specify the signedness of the use
because use signedness does not really make sense:
- if we care about the sign, it should be in the input's type
  (this is DCHECKed).
- if we do not care (e.g., trunctaing word32), then it should not
  be necessary. (And it is upto the user how it interprets the bits.)

Review URL: https://codereview.chromium.org/1462503005

Cr-Commit-Position: refs/heads/master@{#32110}