[func-sig-opts] Add the call SILFunction::spliceBody() to splice a function's body onto another function.

I also added code to ilist_traits so we can set the parent function on BB to be the new function.

Swift SVN r21729

lib/SIL/SILFunction.cpp

@@ -79,6 +79,10 @@ SILFunction::SILFunction(SILModule &Module, SILLinkage Linkage,
     Module.functions.insert(SILModule::iterator(InsertBefore), this);
   else
     Module.functions.push_back(this);
+
+  // Set our BB list to have this function as its parent. This enables us to
+  // splice efficiently basic blocks in between functions.
+  BlockList.Parent = this;
 }
 
 SILFunction::~SILFunction() {