[SimplifyCFG] performBranchToCommonDestFolding(): require block-closed SSA...

[SimplifyCFG] performBranchToCommonDestFolding(): require block-closed SSA form for bonus instructions (PR51125)

I can't seem to wrap my head around the proper fix here,
we should be fine without this requirement, iff we can form this form,
but the naive attempt (https://reviews.llvm.org/D106317) has failed.
So just to unblock the release, put up a restriction.

Fixes https://bugs.llvm.org/show_bug.cgi?id=51125