[llvm] [AMDGPU] Fix sign confusion in performMulLoHiCombine (PR #105831)

[via llvm-commits]

llvmbot wrote:

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Failed to cherry-pick: d0fe52d

https://github.com/llvm/llvm-project/actions/runs/10581862592

Please manually backport the fix and push it to your github fork.  Once this is done, please create a [pull request](https://github.com/llvm/llvm-project/compare)

https://github.com/llvm/llvm-project/pull/105831