tries to match a dead MipsLo node (explanation in the link below). http://article.gmane.org/gmane.comp.compilers.llvm.devel/42757/match=dagcombiner+dead git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@139634 91177308-0d34-0410-b5e6-96231b3b80d8