[llvm] [SelectionDAG] Add `STRICT_BF16_TO_FP` and `STRICT_FP_TO_BF16` (PR #80056)

[Shilei Tian via llvm-commits]

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+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
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shiltian wrote:

This file was adapted from its `half` brother. I removed the `f16c` attribute tests because I guess they will not affect `bfloat`. I commented out those `fpext` tests since we can't support it right now.

https://github.com/llvm/llvm-project/pull/80056