llvm-reduce: improve basic-blocks removal pass

When the single branch target of a block has been removed try updating
it to target a  block that is kept (by scanning forward in the sequence)
instead of replacing the branch with a return instruction. Doing so
reduces the risk of breaking loop structures meaning that when the loop
is 'interesting' these reductions should have more blocks eliminated.

Differential Revision: https://reviews.llvm.org/D125766