In BasicBlock::splitBasicBlock, just use islist::splice to move the instructions,

In BasicBlock::splitBasicBlock, just use islist::splice to move the instructions,
instead of a loop that is really inefficient with large basic blocks.

This speeds up the inliner pass on the testcase in PR209 from 13.8s to 2.24s
which still isn't exactly speedy, but is a lot better.  :)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@11105 91177308-0d34-0410-b5e6-96231b3b80d8