reland: [Local] Allow creating callbr with duplicate successors

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.

Reviewed By: nickdesaulniers

Differential Revision: https://reviews.llvm.org/D129997

Originally landed as
commit 08860f52 ("[Local] Allow creating callbr with duplicate successors")

Reverted in
commit 1cf6b93d ("Revert "[Local] Allow creating callbr with duplicate successors"")