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.cpp

@@ -212,6 +212,7 @@ Target::GetTarget(const char *arch, const char *cpu, const char *isa,
 
     // This is the case for most of them
     t->hasHalf = t->hasRand = t->hasTranscendentals = false;
+    t->hasGather = t->hasScatter = false;
 
     if (!strcasecmp(isa, "sse2")) {
         t->isa = Target::SSE2;