Rollup merge of #156524 - Zalathar:no-pre-codegen, r=oli-obk

Remove the dummy `PreCodegen` mir-opt pass, and use `runtime-optimized` instead

- Alternative to https://github.com/rust-lang/rust/pull/156358.

The `PreCodegen` pass doesn't do anything on its own; it was only serving as a marker to allow `-Zdump-mir` and mir-opt tests to easily dump the final MIR just before codegen.

However, https://github.com/rust-lang/rust/pull/156358#issuecomment-4422445297 pointed out that the `runtime-optimized` phase transition should dump the same MIR, so there shouldn't be any need for a separate *PreCodegen* pass.

---
r? oli-obk (or compiler)