Assert to bounds check MDNode::getOperand.

The getOperandPtr utility already bounds checks, but allows one-off-the-end.
This assert should catch the cases that could previously have been dereferencing
these one-off-the-end pointer. Happily, no cases of this came up with this
change.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@176721 91177308-0d34-0410-b5e6-96231b3b80d8