[BranchRelaxation] Fix invalid branch generation in branch-relaxation (#162065)
If we have MBB with only one successor which is accessable through both
conditional and unconditional branches (TBB == FBB), in
`fixupConditionalBranch` we will first replace FBB with NewMBB in
successors list - `MBB->replaceSuccessor(FBB, NewBB);`, and then create
branch to TBB - `insertBranch(MBB, &NextBB, TBB, Cond);`, ending up with
two branches to different blocks, but only one successor.
Fixes: https://github.com/llvm/llvm-project/issues/162063