Use llvm TargetData to find object sizes, offsets.

Previously, to compute the size of objects and the offsets of struct
elements within structs, we were using the trick of using getelementpointer 
with a NULL base pointer and then casting the result to an int32/64.
However, since we actually know the target we're compiling for at
compile time, we can use corresponding methods from TargetData to
get these values directly.

This mostly cleans up code, but may make some of the gather/scatter
lowering to loads/stores optimizations work better in the presence
of structures.

ispc.h

@@ -160,6 +160,14 @@ struct Target {
     /** Returns a string like "avx" encoding the target. */
     const char *GetISAString() const;
 
+    /** Returns the size of the given type */
+    llvm::Value *SizeOf(LLVM_TYPE_CONST llvm::Type *type);
+    /** Given a structure type and an element number in the structure,
+        returns a value corresponding to the number of bytes from the start
+        of the structure where the element is located. */
+    llvm::Value *StructOffset(LLVM_TYPE_CONST llvm::Type *type,
+                              int element);
+
     /** llvm Target object representing this target. */
     const llvm::Target *target;