Added support for 'uniform' global atomics.

Issue #93.

tests/atomics-1.ispc

@@ -5,7 +5,8 @@ uniform unsigned int32 s = 0;
 
 export void f_f(uniform float RET[], uniform float aFOO[]) {
     float a = aFOO[programIndex]; 
-    float b = atomic_add_global(s, 1);
+    float delta = 1;
+    float b = atomic_add_global(s, delta);
     RET[programIndex] = reduce_add(b);
 }