[llvm] [AArch64][MC] Improve `isSImm` Diagnostic Fidelity (PR #127255)

[Oliver Stannard via llvm-commits]

ostannard wrote:

>From the tests, it looks like this is making the diagnostics worse. Is there some future improvement which this is needed to unblock, or could we change `isImmScaled` to return `NearMatch` in more cases, to get the more informative diagnostic messages back?

https://github.com/llvm/llvm-project/pull/127255