Fix a use of uninitialized memory in SmallVector's move-assignment operator.

Fix a use of uninitialized memory in SmallVector's move-assignment operator.

When we were moving from a larger vector to a smaller one but didn't
need to re-allocate, we would move-assign over uninitialized memory in
the target, then move-construct that same data again.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207663 91177308-0d34-0410-b5e6-96231b3b80d8