Add a very simple cost model to estimate runtime cost of running code.

This is currently only used to decide whether it's worth doing an
"are all lanes running" check at the start of functions--for small
functions, it's not worth the overhead.

The cost is estimated relatively early in compilation (e.g. before
we know if an array access is a scatter/gather or not, before
constant folding, etc.), so there are many known shortcomings.

expr.h

@@ -121,6 +121,7 @@ public:
     void Print() const;
     Expr *Optimize();
     Expr *TypeCheck();
+    int EstimateCost() const;
 
 private:
     const Op op;
@@ -164,6 +165,7 @@ public:
 
     Expr *Optimize();
     Expr *TypeCheck();
+    int EstimateCost() const;
 
 private:
     const Op op;
@@ -196,6 +198,7 @@ public:
 
     Expr *Optimize();
     Expr *TypeCheck();
+    int EstimateCost() const;
 
 private:
     const Op op;
@@ -217,6 +220,7 @@ public:
 
     Expr *Optimize();
     Expr *TypeCheck();
+    int EstimateCost() const;
 
 private:
     Expr *test, *expr1, *expr2;
@@ -240,6 +244,7 @@ public:
     llvm::Constant *GetConstant(const Type *type) const;
     ExprList *Optimize();
     ExprList *TypeCheck();
+    int EstimateCost() const;
 
     std::vector<Expr *> exprs;
 };
@@ -257,6 +262,7 @@ public:
 
     Expr *Optimize();
     Expr *TypeCheck();
+    int EstimateCost() const;
 
 private:
     Expr *func;
@@ -285,6 +291,7 @@ public:
 
     Expr *Optimize();
     Expr *TypeCheck();
+    int EstimateCost() const;
 
 private:
     Expr *arrayOrVector, *index;
@@ -303,13 +310,15 @@ public:
     MemberExpr(Expr *expr, const char *identifier, SourcePos pos, 
                SourcePos identifierPos);
 
-    virtual llvm::Value *GetValue(FunctionEmitContext *ctx) const;
-    virtual llvm::Value *GetLValue(FunctionEmitContext *ctx) const;
-    virtual const Type *GetType() const;
-    virtual Symbol *GetBaseSymbol() const;
-    virtual void Print() const;
-    virtual Expr *Optimize();
-    virtual Expr *TypeCheck();
+    llvm::Value *GetValue(FunctionEmitContext *ctx) const;
+    llvm::Value *GetLValue(FunctionEmitContext *ctx) const;
+    const Type *GetType() const;
+    Symbol *GetBaseSymbol() const;
+    void Print() const;
+    Expr *Optimize();
+    Expr *TypeCheck();
+    int EstimateCost() const;
+
     virtual int getElementNumber() const;
 
 protected:
@@ -392,6 +401,7 @@ public:
 
     Expr *TypeCheck();
     Expr *Optimize();
+    int EstimateCost() const;
 
     /** Return the ConstExpr's values as booleans, doing type conversion
         from the actual type if needed.  If forceVarying is true, then type
@@ -495,6 +505,7 @@ public:
     void Print() const;
     Expr *TypeCheck();
     Expr *Optimize();
+    int EstimateCost() const;
 
 private:
     const Type *type;
@@ -514,6 +525,7 @@ public:
     void Print() const;
     Expr *TypeCheck();
     Expr *Optimize();
+    int EstimateCost() const;
 
 private:
     Expr *expr;
@@ -533,6 +545,7 @@ public:
     void Print() const;
     Expr *TypeCheck();
     Expr *Optimize();
+    int EstimateCost() const;
 
 private:
     Expr *expr;
@@ -551,6 +564,7 @@ public:
     Expr *TypeCheck();
     Expr *Optimize();
     void Print() const;
+    int EstimateCost() const;
 
 private:
     Symbol *symbol;
@@ -571,6 +585,7 @@ public:
     Expr *TypeCheck();
     Expr *Optimize();
     void Print() const;
+    int EstimateCost() const;
 
 private:
     friend class FunctionCallExpr;
@@ -597,6 +612,7 @@ public:
     Expr *TypeCheck();
     Expr *Optimize();
     void Print() const;
+    int EstimateCost() const;
 };
 
 #endif // ISPC_EXPR_H