[llvm] [AArch64] Check for negative numbers when adjusting icmps (PR #140999)

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AZero13 wrote:

> Your title says `isLegalICmpImmediate` which is the name of a function in TargetTransformInfo and TargetLowering, but this patch doesn't use that function.

It was this way at first but then it didn't compile so I did this instead.

https://github.com/llvm/llvm-project/pull/140999