Add allnodes() iterator range to SelectionDAG. NFC.

Add allnodes() iterator range to SelectionDAG.  NFC.

SelectionDAG already had begin/end methods for iterating over all
the nodes, but didn't define an iterator_range for us in foreach
loops.

This adds such a method and uses it in some of the eligible places
throughout the backends.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@242212 91177308-0d34-0410-b5e6-96231b3b80d8