[PowerPC] Fix the incorrect iterator inside peephole

Instruction selection can insert nodes into the underlying list after the root
node so iterating will thereby miss it. We should NOT assume that, the root node
is the last element in the DAG nodelist.

Patch by: steven.zhang (Qing Shan Zhang)

Differential Revision: https://reviews.llvm.org/D47437

llvm-svn: 333415