AnalyzeBranch doesn't change which successors a bb has, just the order

AnalyzeBranch doesn't change which successors a bb has, just the order
we try to branch to them.

Before we were creating successor lists with duplicated entries. Fixing that
found a bug in isBlockOnlyReachableByFallthrough that would causes it to
return the wrong answer for

-----------
...
jne foo
jmp bar

foo:
----------

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@132882 91177308-0d34-0410-b5e6-96231b3b80d8