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
diff --git a/src/IceCfgNode.cpp b/src/IceCfgNode.cpp
index 3fd63e2..2f2897c 100644
--- a/src/IceCfgNode.cpp
+++ b/src/IceCfgNode.cpp
@@ -235,8 +235,9 @@
   // with the sentinel instruction number 0.
   std::vector<InstNumberT> &LiveBegin = Liveness->getLiveBegin(this);
   std::vector<InstNumberT> &LiveEnd = Liveness->getLiveEnd(this);
-  LiveBegin.assign(NumVars, Inst::NumberSentinel);
-  LiveEnd.assign(NumVars, Inst::NumberSentinel);
+  InstNumberT Sentinel = Inst::NumberSentinel;
+  LiveBegin.assign(NumVars, Sentinel);
+  LiveEnd.assign(NumVars, Sentinel);
   // Initialize Live to be the union of all successors' LiveIn.
   for (NodeList::const_iterator I = OutEdges.begin(), E = OutEdges.end();
        I != E; ++I) {