Always generate GEP i8 / ptradd for struct offsets This implements #98615, and goes a bit further to remove `struct_gep` entirely. Upstream LLVM is in the beginning stages of [migrating to `ptradd`](https://discourse.llvm.org/t/rfc-replacing-getelementptr-with-ptradd/68699). LLVM 19 will [canonicalize](https://github.com/llvm/llvm-project/pull/68882) all constant-offset GEPs to i8, which has roughly the same effect as this change. Fixes #121719. Split out from #121577. r? `@nikic`