Rearrange operands of the BranchInst, to be able to

Rearrange operands of the BranchInst, to be able to
access each with a fixed negative index from op_end().

This has two important implications:
- getUser() will work faster, because there are less iterations
  for the waymarking algorithm to perform. This is important
  when running various analyses that want to determine callers
  of basic blocks.
- getSuccessor() now runs faster, because the indirection via OperandList
  is not necessary: Uses corresponding to the successors are at fixed
  offset to "this".

The price we pay is the slightly more complicated logic in the operator
User::delete, as it has to pick up the information whether it has to free
the memory of an original unconditional BranchInst or a BranchInst that
was originally conditional, but has been shortened to unconditional.
I was not able to come up with a nicer solution to this problem. (And
rest assured, I tried *a lot*).

Similar reorderings will follow for InvokeInst and CallInst. After that
some optimizations to pred_iterator and CallSite will fall out naturally.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@66815 91177308-0d34-0410-b5e6-96231b3b80d8