Merge r168147 from trunk:

  Constant::IsThreadDependent(): Use dyn_cast<Constant> instead of cast

  It turns out that the operands of a Constant are not always themselves
  Constant. For example, one of the operands of BlockAddress is
  BasicBlock, which is not a Constant.

  This should fix the dragonegg-x86_64-linux-gcc-4.6-test build which
  broke in r168037.

llvm-svn: 168193