Incorporate per-lane offsets for varying data in the front-end.

Previously, it was only in the GatherScatterFlattenOpt optimization pass that
we added the per-lane offsets when we were indexing into varying data.
(Specifically, the case of float foo[]; int index; foo[index], where foo
is an array of varying elements rather than uniform elements.)  Now, this
is done in the front-end as we're first emitting code.

In addition to the basic ugliness of doing this in an optimization pass, 
it was also error-prone to do it there, since we no longer have access
to all of the type information that's around in the front-end.

No functionality or performance change.

ctx.h

@@ -525,6 +525,7 @@ private:
                         const Type *type, const char *name);
     void maskedStore(llvm::Value *rvalue, llvm::Value *lvalue,
                      const Type *rvalueType, llvm::Value *maskPtr);
+    llvm::Value *addVaryingOffsetsIfNeeded(llvm::Value *value, const Type *type);
 };
 
 #endif // ISPC_CTX_H