Handle more instruction types when flattening offset vectors.

Generalize the lScalarizeVector() utility routine (used in determining
when we can change gathers/scatters into vector loads/stores, respectively)
to handle vector shuffles and vector loads.  This fixes issue #79, which
provided a case where a gather was being performed even though a vector
load was possible.

tests/shuffle-flatten.ispc

@@ -0,0 +1,17 @@
+
+export uniform int width() { return programCount; }
+
+export void f_f(uniform float RET[], uniform float aFOO[]) {
+    int tmp1 = shuffle(programIndex, 0, programIndex);
+
+    RET[programIndex] = 10;
+    if (programIndex < 1) {
+        uniform int foo = extract(tmp1, 0);
+        RET[programIndex] = aFOO[foo + programIndex];
+    }
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = 10;
+    RET[0] = 1;
+}