Give CloneBasicBlock an optional function argument to specify which function

Give CloneBasicBlock an optional function argument to specify which function
to add the cloned block to.  This allows the block to be added to the function
immediately, and all of the instructions to be immediately added to the function
symbol table, which speeds up the inliner from 3.7 -> 3.38s on the PR209.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@11107 91177308-0d34-0410-b5e6-96231b3b80d8