Fix branch folding bug for indirect branches: for a block containing only

Fix branch folding bug for indirect branches: for a block containing only
an unconditional branch (possibly from tail merging), this code is
trying to redirect all of its predecessors to go directly to the branch
target, but that isn't feasible for indirect branches.  The other
predecessors (that don't end with indirect branches) could theoretically
still be handled, but that is not easily done right now.

The AnalyzeBranch interface doesn't currently let us distinguish jump table
branches from indirect branches, and this code is currently handling
jump tables.  To avoid punting on address-taken blocks, we would have to give
up handling jump tables.  That seems like a bad tradeoff.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@85975 91177308-0d34-0410-b5e6-96231b3b80d8