[SimplifyCFG] Thread all predecessors with same value at once

If there are multiple predecessors that have the same condition
value (and thus same "real destination"), these were previously
handled by copying the threaded block for each predecessor.
Instead, we can reuse one block for all of them. This makes the
behavior of SimplifyCFG's jump threading match that of the
actual JumpThreading pass.

This also avoids the infinite combine loop reported in:
https://reviews.llvm.org/D124159#3624387