Auto merge of #121665 - erikdesjardins:ptradd, r=nikic

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](llvm/llvm-project#68882) all constant-offset GEPs to i8, which has roughly the same effect as this change.

Fixes #121719.

Split out from #121577.

r? `@nikic`

Author: bors