do not bother reuniquing mdnodes whose operands drop to null. Doing

do not bother reuniquing mdnodes whose operands drop to null.  Doing
so can be a huge performance issue when tearing down modules and mdnodes
are not guaranteed to be unique anyway.  This speeds up:
$ time ~/llvm/Release/bin/clang gcc.c -w -S -g

from 72 to 35s, where gcc.c is from:
http://people.csail.mit.edu/smcc/projects/single-file-programs/

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@92315 91177308-0d34-0410-b5e6-96231b3b80d8