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

Review URL: https://codereview.chromium.org/311243006

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