Use logical shift right op for shifts of unsigned ints. Fixes issue #88.

tests/uint-shift.ispc

@@ -0,0 +1,11 @@
+
+export uniform int width() { return programCount; }
+
+export void f_f(uniform float RET[], uniform float aFOO[]) {
+    unsigned int x = 0xffffffff;
+    RET[programIndex] = x >> 16; 
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = 0xffff;
+}