[llvm] [LoongArch] Pre-commit tests for absolute difference (PR #132898)

[Lu Weining via llvm-commits]

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SixWeining wrote:

Leave a TODO or FIXME in this file explaining why you add this file. For example:
;; TODO: Currently LoongArch generates sub-optimal code for these tests (the `xxx` instruction can be omitted or changed to `yyy`). Later patch will address it.

https://github.com/llvm/llvm-project/pull/132898