Revert "[SimplifyCFG] Update FoldBranchToCommonDest to be poison-safe"

This reverts commit 99108c79.
Clang is miscompiling LLVM with this change, a stage-2 build hits
multiple failures.

As a repro, I built clang in a stage1 directory and used it this way:

cmake -G Ninja ../llvm \
  -DCMAKE_CXX_COMPILER=`pwd`/../build-stage1/bin/clang++ \
  -DCMAKE_C_COMPILER=`pwd`/../build-stage1/bin/clang \
  -DLLVM_TARGETS_TO_BUILD="X86;NVPTX;AMDGPU" \
  -DLLVM_ENABLE_PROJECTS=mlir \
  -DLLVM_BUILD_EXAMPLES=ON \
  -DCMAKE_BUILD_TYPE=Release \
  -DLLVM_ENABLE_ASSERTIONS=On
ninja check-mlir