Improve gather->vector load optimization to detect <linear sequence>-<uniform> case.

Previously, we didn't handle subtraction ops when deciphering offsets in order to
try to change gathers t evictor loads.

tests/gather-to-vload-neg-offset.ispc

@@ -0,0 +1,13 @@
+
+
+export uniform int width() { return programCount; }
+
+export void f_fu(uniform float RET[], uniform float aFOO[], uniform float b) {
+    float a = aFOO[programIndex]; 
+    int index = programIndex + 5;
+    RET[index-b] = a; 
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = 1 + programIndex;
+}