[llvm] [CodeGen] Use byte offsets and ptradd in ShadowStackGCLowering (PR #178436)

[Jameson Nash via llvm-commits]

vtjnash wrote:

I also recommend you also just write your own gc roots layout pass, since this one is not all that great (and has many limitations like exactly that baked in the design)

https://github.com/llvm/llvm-project/pull/178436