Since D129288, callbr is allowed to have duplicate successors. This patch removes a limitation which prevents optimizations from actually producing such callbrs.
This is probably the riskiest of all the recent callbr changes, because code with incorrect assumptions might be lurking somewhere. I fixed the one case I encountered ahead of time in https://github.com/llvm/llvm-project/commit/8201e3ef5c84561260218bc041209611aac690e3.