Avoid comparing invalid slot indexes, and assert that it doesn't happen.

Avoid comparing invalid slot indexes, and assert that it doesn't happen.

The SlotIndex created by the default construction does not represent a position
in the function, and it doesn't make sense to compare it to other indexes.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@126924 91177308-0d34-0410-b5e6-96231b3b80d8