Rollup merge of #122320 - erikdesjardins:vtable, r=nikic

Use ptradd for vtable indexing

Extension of #121665.

After this, the only remaining usages of GEP are [this](https://github.com/rust-lang/rust/blob/cd81f5b27ee00b49d413db50b5e6af871cebcf23/compiler/rustc_codegen_llvm/src/intrinsic.rs#L909-L920) kinda janky Emscription EH code, which I'll change in a future PR, and array indexing / pointer offsets, where there isn't yet a canonical `ptradd` form. (Out of curiosity I tried converting the latter to `ptradd(ptr, mul(size, index))`, but that causes codegen regressions right now.)

r? `@nikic`

Author: workingjubilee