Fix some bugs in SimplifyNodeWithTwoResults where it would call deletenode to

Fix some bugs in SimplifyNodeWithTwoResults where it would call deletenode to
delete a node even if it was not dead in some cases.  Instead, just add it to
the worklist.  Also, make sure to use the CombineTo methods, as it was doing
things that were unsafe: the top level combine loop could touch dangling memory.

This fixes CodeGen/Generic/2008-01-25-dag-combine-mul.ll

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@46384 91177308-0d34-0410-b5e6-96231b3b80d8