[amdgpu] Fix a crash case when preserving MDT in SILowerControlFlow

- When a redundant MBB is being erased from MDT, check whether its
  single successor is dominiated by it. If yes, update that successor's
  idom before erasing MBB; otherwise, it implies MBB is a leaf node and
  could be erased directly.

Reviewed By: foad

Differential Revision: https://reviews.llvm.org/D111831