Propagate DAG node ordering during type legalization and instruction selection

A node's ordering is only propagated during legalization if (a) the new node does
not have an ordering (is not a CSE'd node), or (b) the new node has an ordering
that is higher than the node being legalized.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@177465 91177308-0d34-0410-b5e6-96231b3b80d8