Bugfix for non-const shift amt and unit tests.

tests/shift-1.ispc

@@ -0,0 +1,14 @@
+
+export uniform int width() { return programCount; }
+
+export void f_fu(uniform float RET[], uniform float aFOO[], uniform float b) {
+    int a = aFOO[programIndex]; 
+    int rot = shift(a, -1);
+    RET[programIndex] = rot;
+}
+
+export void result(uniform float RET[]) {
+    varying int val = programIndex;
+    if (val < 0) val = 0;	 
+    RET[programIndex] = val;	 
+}