Added support for 'uniform' global atomics.

Issue #93.

tests/atomics-uniform-7.ispc

@@ -0,0 +1,14 @@
+
+export uniform int width() { return programCount; }
+
+uniform float s = 100.;
+
+export void f_f(uniform float RET[], uniform float aFOO[]) {
+    float a = aFOO[programIndex]; 
+    uniform float b = atomic_swap_global(s, 1.);
+    RET[programIndex] = b;
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = 100.;
+}