[X86] In PostprocessISelDAG, start from allnodes_end, not the root.

There is no guarantee the root is at the end if isel created any nodes without morphing them. This includes the nodes created by manual isel from C++ code in X86ISelDAGToDAG.

This is similar to r333415 from PowerPC which is where I originally stole the peephole loop from.

I don't have a test case, but without this a future patch doesn't work which is how I found it.

llvm-svn: 344808