Implement the code for PowerPC/inverted-bool-compares.ll, even though it

Implement the code for PowerPC/inverted-bool-compares.ll, even though it
that testcase still does not pass with the dag combiner.  This is because
not all forms of br* are folded yet.

Also, when we combine a node into another one, delete the node immediately
instead of waiting for the node to potentially come up in the future.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@23632 91177308-0d34-0410-b5e6-96231b3b80d8