[PowerPC] Fix a DAG replacement bug in PPCTargetLowering::DAGCombineExtBoolTrunc

While promoting nodes in PPCTargetLowering::DAGCombineExtBoolTrunc, it is
possible for one of the nodes to be replaced by another. To make sure we do not
visit the deleted nodes, and to make sure we visit the replacement nodes, use a
list of HandleSDNodes to track the to-be-promoted nodes during the promotion
process.

The same fix has been applied to the analogous code in
PPCTargetLowering::DAGCombineTruncBoolExt.

Fixes PR26985.

llvm-svn: 269272