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.

ispc.h

@@ -148,6 +148,8 @@ public:
         pointer in place of the original ASTNode *. */
     virtual ASTNode *TypeCheck() = 0;
 
+    virtual int EstimateCost() const = 0;
+
     /** All AST nodes must track the file position where they are
         defined. */
     const SourcePos pos;
@@ -365,6 +367,25 @@ struct Globals {
     std::vector<std::string> cppArgs;
 };
 
+enum {
+    COST_FUNCALL = 4,
+    COST_TASK_LAUNCH = 16,
+    COST_SELECT = 4,
+    COST_RETURN = 4,
+    COST_SIMPLE_ARITH_LOGIC_OP = 1,
+    COST_COMPLEX_ARITH_OP = 4,
+    COST_COHERENT_BREAK_CONTINE = 4,
+    COST_REGULAR_BREAK_CONTINUE = 2,
+    COST_UNIFORM_LOOP = 4,
+    COST_VARYING_LOOP = 6,
+    COST_SYNC = 32,
+    COST_LOAD = 2,
+    COST_DEREF = 4,
+    COST_TYPECAST_SIMPLE = 1,
+    COST_TYPECAST_COMPLEX = 4,
+    COST_GATHER = 8
+};
+
 extern Globals *g;
 extern Module *m;