IR: Add BasicBlock::insertInto()

IR: Add BasicBlock::insertInto()

Although unlinked `BasicBlock`s can be created, there's currently no way
to insert them into `Function`s after the fact.  In particular,
`moveAfter()` and `moveBefore()` require that the basic block is already
linked.

Extract the logic for initially linking a `BasicBlock` out of the
constructor and into a member function that can be used for lazy
insertion.

  - Asserts that the basic block is currently unlinked.
  - Matches the logic of the constructor.
  - Changed the constructor to use it since the logic matches.

This is needed in a follow-up commit for PR5680.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214563 91177308-0d34-0410-b5e6-96231b3b80d8