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

[Sam Elliott via llvm-commits]

lenary 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?

If you look at the full test files, the implementer of the diagnostics made a specific choice to give "index must be in range [...]" errors only when an index/immediate has been provided - and to just give "invalid operand ..." when the user provided something that's not an immediate (in these tests, a register). This is how the `isSImmScaled` existing diagnostics work right now.

I'm not bothered either way, this is just something I noticed. I'm not spending several months refactoring the AArch64 assembler error messages, I only got here because I wanted to start using DiagnosticPredicates for RISC-V. 

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