dead branch elim: Track killed backedges

dead branch elim: Track killed backedges

When deleting branches and blocks, also remove them from
the backedges set, in case they were there.

This prevents us from keeping stale pointers to deleted Instruction
objects.  That memory could be used later by another instruction,
incorrectly signaling that something has a backedge reference, and
the dead branch eliminator could end up deleting live blocks.

Adds accessor method ir::BasicBlock::terminator

Fixes https://github.com/KhronosGroup/SPIRV-Tools/issues/1168