[llvm] [MachineBasicBlock] Don't split loop header successor if the terminator is unanalyzable (PR #170146)

[Hongyu Chen via llvm-commits]

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+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 6
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XChy wrote:

Add a MIR test now. The reproduction requires the `requiresStructuredCFG` feature, so the generic test may not be suitable. 

https://github.com/llvm/llvm-project/pull/170146