Add foreach and foreach_tiled looping constructs

These make it easier to iterate over arbitrary amounts of data
elements; specifically, they automatically handle the "ragged
extra bits" that come up when the number of elements to be
processed isn't evenly divided by programCount.

TODO: documentation

ctx.h

@@ -159,6 +159,10 @@ public:
         finished. */
     void EndLoop();
 
+    /** */
+    void StartForeach();
+    void EndForeach();
+
     /** Emit code for a 'break' statement in a loop.  If doCoherenceCheck
         is true, then if we're in a 'varying' loop, code will be emitted to
         see if all of the lanes want to break, in which case a jump to the
@@ -183,6 +187,8 @@ public:
         flow */
     int VaryingCFDepth() const;
 
+    bool InForeachLoop() const;
+
     /** Called to generate code for 'return' statement; value is the
         expression in the return statement (if non-NULL), and
         doCoherenceCheck indicates whether instructions should be generated