Modify SizeOf() and StructOffset() to not compute value based on target for generic targets.

Specifically, we want to be able to late-bind on whether the mask is i32s or i1s, so if there's
any chance of ambiguity, we emit code that does the "GEP from a NULL base pointer" trick to
compute the value later in compilation.

ispc.h

@@ -177,12 +177,14 @@ struct Target {
     const char *GetISAString() const;
 
     /** Returns the size of the given type */
-    llvm::Value *SizeOf(LLVM_TYPE_CONST llvm::Type *type);
+    llvm::Value *SizeOf(LLVM_TYPE_CONST llvm::Type *type,
+                        llvm::BasicBlock *insertAtEnd);
+
     /** 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);
+                              int element, llvm::BasicBlock *insertAtEnd);
 
     /** llvm Target object representing this target. */
     const llvm::Target *target;