[SimplifyCFG][TranformUtils]Do not simplify away a trivial basic block if both this block and at least one of its predecessors are loop latches.

- Before this patch, loop metadata (if exists) will override the metadata of each predecessor; if the predecessor block already has loop metadata, the orignal loop metadata won't be preserved and could cause missed loop transformations (see 'test2' in llvm/test/Transforms/SimplifyCFG/preserve-llvm-loop-metadata.ll).

To illustrate how inner-loop metadata might be dropped before this patch:

CFG Before

      entry
        |
        v
 ---> while.cond   ------------->  while.end
 |       |
 |       v
 |   while.body
 |       |
 |       v
 |    for.body <---- (md1)
 |       |  |______|
 |       v
 |    while.cond.exit (md2)
 |       |
 |_______|

CFG After

       entry
         |
         v
 ---> while.cond.rewrite  ------------->  while.end
 |       |
 |       v
 |   while.body
 |       |
 |       v
 |    for.body <---- (md2)
 |_______|  |______|

Basically, when 'while.cond.exit' is folded into 'while.cond', 'md2' overrides 'md1' and 'md1' is dropped from the CFG.

Differential Revision: https://reviews.llvm.org/D134152

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