Issue warning when compile-time constant out-of-bounds array index is used.

Issue #98.
Also fixes two examples that had bugs of this type that this warning
  uncovered!

tests/array-scatter-vary.ispc

@@ -13,7 +13,7 @@ export void f_fu(uniform float RET[], uniform float aFOO[], uniform float b) {
 }
 
     
-export void result(uniform float RET[4]) { 
+export void result(uniform float RET[]) { 
     RET[programIndex] = 0;
     RET[3] = 4;
     RET[4] = 5;