[SDAG] Allow the legalizer to delete an illegally typed intermediate

introduced during legalization. This pattern is based on other patterns
in the legalizer that I changed in the same way. Now, the legalizer
eagerly collects its garbage when necessary so that we can survive
leaving such nodes around for it.

Instead, we add an assert to make sure the node will be correctly
handled by that layer.

llvm-svn: 214602