Add assert() statement support. Issue #106.

tests/assert-4.ispc

@@ -0,0 +1,16 @@
+
+
+export uniform int width() { return programCount; }
+
+export void f_fu(uniform float RET[], uniform float aFOO[], uniform float b) {
+    float a = aFOO[programIndex]; 
+    if (a < 0)
+        // Though this condition is false, the assert shouldn't hit, since
+        // given the if test above, all lanes should be off.
+        assert(b == 0.);
+    RET[programIndex] = a; 
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = 1 + programIndex;
+}