Add support for non-factored variants of gather/scatter functions.

We now have two ways of approaching gather/scatters with a common base
pointer and with offset vectors.  For targets with native gather/scatter,
we just turn those into base + {1/2/4/8}*offsets.  For targets without,
we turn those into base + {1/2/4/8}*varying_offsets + const_offsets,
where const_offsets is a compile-time constant.

Infrastructure for issue #325.

ispc.h

@@ -252,9 +252,15 @@ struct Target {
         conversions. */
     bool hasHalf;
 
-    /** Indicates whether there is an ISA random number instruciton. */
+    /** Indicates whether there is an ISA random number instruction. */
     bool hasRand;
 
+    /** Indicates whether the target has a native gather instruction */
+    bool hasGather;
+
+    /** Indicates whether the target has a native scatter instruction */
+    bool hasScatter;
+
     /** Indicates whether the target has support for transcendentals (beyond
         sqrt, which we assume that all of them handle). */
     bool hasTranscendentals;