[AMDGPU] SILowerControlFlow::removeMBBifRedundant should not try to change MBB...

[AMDGPU] SILowerControlFlow::removeMBBifRedundant should not try to change MBB layout if it can fallthrough

removeMBBifRedundant normally tries to keep predecessors fallthrough when removing redundant MBB.
         It has to change MBBs layout to keep the new successor to immediately follow the predecessor of removed MBB.
         It only may be allowed in case the new successor itself has no successors to which it fall through.

Reviewed By: rampitec

Differential Revision: https://reviews.llvm.org/D89397