[clang] [llvm] [AArch64] Support optional constant offset for constraint "S" (PR #80255)

[Fangrui Song via cfe-commits]

MaskRay wrote:

> Just to make sure I've understood:
> 
> * x86_64's support for constraint "s" was hoisted into the target independent TargetLowering in [reviews.llvm.org/D61560](https://reviews.llvm.org/D61560)
> * Clang's implementation of constraint "s" isn't restricted to non-PIC/non-PIE like GCC's
> * We can implement constraint "S" in terms of constraint "s" as Clang's implementation of "s" is equivalent to AArch64 "S".

Yes, these are all correct. 

> On that basis this looks reasonable to me. I've not had time to pick over the details of the code/test changes though.

Thanks!

https://github.com/llvm/llvm-project/pull/80255