Fix a really nasty caching bug I introduced in memdep. An entry

was being added to the Result vector, but not being put in the
cache.  This means that if the cache was reused wholesale for a
later query that it would be missing this entry and we'd do an
incorrect load elimination.

Unfortunately, it's not really possible to write a useful 
testcase for this, but this unbreaks 255.vortex.

llvm-svn: 90093