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

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@11116 91177308-0d34-0410-b5e6-96231b3b80d8