fix crash in SmallDenseMap copy constructor

Prevent a crash in the SmallDenseMap copy constructor whenever the other
map is not in small mode.

<rdar://problem/14292693>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202206 91177308-0d34-0410-b5e6-96231b3b80d8