DAGCombiner: Don't fold xor into not if getNOT would introduce an illegal constant.

This happens e.g. with <2 x i64> -1 on x86_32. It cannot be generated directly
because i64 is illegal. It would be nice if getNOT would handle this
transparently, but I don't see a way to generate a legal constant there right
now. Fixes PR17487.

llvm-svn: 192795