[MS] Apply `inreg` to AArch64 sret parms on instance methods

The documentation rules indicate that instance methods should return
large, trivially copyable aggregates via X1/X0 and not X8 as is normally
done when returning such structs from free functions:
https://docs.microsoft.com/en-us/cpp/build/arm64-windows-abi-conventions?view=vs-2019#return-values

Fixes PR47836, a bug in the initial implementation of these rules.

I tried to simplify the logic a bit as well while I'm here.

Differential Revision: https://reviews.llvm.org/D89362