Insert two blank SlotIndexes between basic blocks instead of just one.

This is the first small step towards using closed intervals for liveness instead
of the half-open intervals we're using now.

We want to be able to distinguish between a SlotIndex that represents a variable
being live-out of a basic block, and an index representing a variable live-in to
its successor.

That requires two separate indexes between blocks. One for live-outs and one for
live-ins.

With this change, getMBBEndIdx(MBB).getPrevSlot() becomes stable so it stays
greater than any instructions inserted at the end of MBB.

llvm-svn: 118747