Performance improvements for global min/max atomics. Issue #57.

Compute a "local" min/max across the active program instances and 
  then do a single atomic memory op.
Added a few tests to exercise global min/max atomics (which were
  previously untested!)

tests/atomics-7.ispc

@@ -0,0 +1,14 @@
+
+export uniform int width() { return programCount; }
+
+uniform int64 s = 0;
+
+export void f_f(uniform float RET[], uniform float aFOO[]) {
+    float a = aFOO[programIndex]; 
+    float b = atomic_add_global(s, 1);
+    RET[programIndex] = reduce_add(b);
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = reduce_add(programIndex);
+}

tests/atomics-8.ispc

@@ -0,0 +1,14 @@
+
+export uniform int width() { return programCount; }
+
+uniform int32 s = 0;
+
+export void f_f(uniform float RET[], uniform float aFOO[]) {
+    int32 a = aFOO[programIndex]; 
+    float b = atomic_min_global(s, a);
+    RET[programIndex] = reduce_min(b);
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = reduce_min(programIndex);
+}