Optimize the case where we are inlining a function that contains only one basic block,

and that basic block ends with a return instruction.  In this case, we can just splice
the cloned "body" of the function directly into the source basic block, avoiding a lot
of rearrangement and splitBasicBlock's linear scan over the split block.  This speeds up
the inliner on the testcase in PR209 from 2.3s to 1.7s, a 35% reduction.

llvm-svn: 11116