Add foreach_unique iteration construct.

Idea via Ingo Wald / IVL compiler.

tests/foreach-unique-2.ispc

@@ -0,0 +1,15 @@
+
+export uniform int width() { return programCount; }
+
+export void f_f(uniform float RET[], uniform float aFOO[]) {
+    uniform int count = 0;
+    float a = aFOO[programIndex];
+    // make sure a++ only evaluated once
+    foreach_unique (ua in a++)
+        ++count;
+    RET[programIndex] = a; 
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = programIndex + 2;
+}