[SimplifyCFG] Don't widen cond br if false branch has successors

Fixes https://github.com/llvm/llvm-project/issues/57221.

This limits the tryWidenCondBranchToCondBranch transform making it
work only if the false block of widenable condition branch
has no successors.

If that block has successors, then SimplifyCondBranchToCondBranch
may undo the transform done by tryWidenCondBranchToCondBranch, which
would lead to infinite cycle of transformation and eventually
an assert failing.

Differential Revision: https://reviews.llvm.org/D132356