If we're only adding one new element to 'Cache', insert it into its known

If we're only adding one new element to 'Cache', insert it into its known
position instead of using a full sort. This speeds up GVN by ~4% with the
new memdep stuff.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@60746 91177308-0d34-0410-b5e6-96231b3b80d8