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

[Jameson Nash via llvm-commits]

vtjnash wrote:

I've also previously filed an issue that is a mistake in the verifier and that particular check needs to be deleted entirely: https://discourse.llvm.org/t/rfc-plan-for-eliminating-allocainst-getallocatedtype-leaving-only-getarraysize/89969. It is just currently of very low importance on my list to fix the verifier, since all it would do is allow slightly more IR to pass there. This PR is a step in the direction of the planned eventual deprecation of getAllocatedType, although I don't represent there is any timeline for it when that will be finished.

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