[SDAG] MorphNodeTo recursively deletes dead operands of the old

[SDAG] MorphNodeTo recursively deletes dead operands of the old
fromulation of the node, which isn't really the desired behavior from
within the combiner or legalizer, but is necessary within ISel. I've
added a hopefully helpful comment and fixed the only two places where
this took place.

Yet another step toward the combiner and legalizer not needing to use
update listeners with virtual calls to manage the worklists behind
legalization and combining.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214574 91177308-0d34-0410-b5e6-96231b3b80d8