If we are extracting a basic block that ends in an invoke call, we must also

If we are extracting a basic block that ends in an invoke call, we must also
extract the landing pad block. Otherwise, there will be a situation where the
invoke's unwind edge lands on a non-landing pad.

We also forbid the user from extracting the landing pad block by itself. Again,
this is not a valid transformation.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@140083 91177308-0d34-0410-b5e6-96231b3b80d8