Add patterns to better-match code generated when accessing SOA data.

In particular, LLVMVectorIsLinear() and LLVMVectorValuesAllEqual() are able
to reason a bit about the effects of the shifts and the ANDs that are
generated from SOA indexing calculations, so that they can detect more cases
where a linear sequence of locations are in fact being accessed in
the presence of SOA data.

tests/soa-27.ispc

@@ -0,0 +1,25 @@
+
+struct Point { float x, y, z; };
+
+export uniform int width() { return programCount; }
+
+export void f_fu(uniform float RET[], uniform float aFOO[], uniform float b) {
+    float a = aFOO[programIndex]; 
+
+    soa<8> Point pts[10];
+
+    foreach (i = 1 ... 80) {
+        pts[i].x = b*i;
+        pts[i].y = 2*b*i;
+        pts[i].z = 3*b*i;
+    }
+    pts[0].x = pts[0].y = pts[0].z = 0;
+
+    uniform Point up = pts[4];
+
+    RET[programIndex] = up.z;
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = 60;
+}