check that terminators only occur at the end of a block. This catches the

check that terminators only occur at the end of a block.  This catches the
common problem of putting two terminators in the same block.  I can't write
a testcase for this because the .ll parser rejects this before the verifier
can, but this can occur when generating IR.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@46900 91177308-0d34-0410-b5e6-96231b3b80d8