Fix a C++ violation

Fix a C++ violation.

Ice::Inst::NumberSentinel is defined within the Inst class definition:

class Inst {
  ...
  static const InstNumberT NumberDeleted = -1;
  static const InstNumberT NumberSentinel = 0;
  ...
};

Under some compilers/options, this causes a link error when passing NumberSentinel as a const T& argument.

(Another option would be to move the actual definitions into IceInst.cpp.)

BUG= none
R=jfb@chromium.org

Committed: https://gerrit.chromium.org/gerrit/gitweb?p=native_client/pnacl-subzero.git;a=commit;h=ab8242c

[JF, 2014-06-05 14:34:14]

lgtm

[Jim Stichnoth, 2014-06-05 15:06:16]

Committed patchset #1 manually as rab8242c (presubmit successful).