ARM: avoid duplicating branches during constant islands.

ARM: avoid duplicating branches during constant islands.

We were using a naive heuristic to determine whether a basic block already had
an unconditional branch at the end. This mostly corresponded to reality
(assuming branches got optimised) because there's not much point in a branch to
the next block, but could go wrong.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221904 91177308-0d34-0410-b5e6-96231b3b80d8