Change the interface to the type legalization method

Change the interface to the type legalization method
ReplaceNodeResults: rather than returning a node which
must have the same number of results as the original
node (which means mucking around with MERGE_VALUES,
and which is also easy to get wrong since SelectionDAG
folding may mean you don't get the node you expect),
return the results in a vector.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@60348 91177308-0d34-0410-b5e6-96231b3b80d8