compiler: Don't mark `SingleUseConsts` MIR pass as "required for soundness"
I don't think this MIR pass is required for soundness. The reasons are:
* Something like it was not enabled by default before PR rust-lang/rust#107404 which was the precursor to `SingleUseConsts` (see rust-lang/rust#125910 for the switch).
* By following the advice from https://github.com/rust-lang/rust/pull/128657#discussion_r1705114015 we can conclude it is not required for soundness since it has only ever run on MIR opt level > 0.
* Its [`MirPass::can_be_overridden()`](0ee7d96253/compiler/rustc_mir_transform/src/pass_manager.rs (L98-L102)) is unchanged and thus returns `true`, indicating that it is not a required MIR pass.
* PR CI pass in rust-lang/rust#151426 which stops enabling it by default in non-optimized builds.
As shown in the updated test `tests/mir-opt/optimize_none.rs`, `#[optimize(none)]` functions become even less optimized, as expected and desired.
Unblocks https://github.com/rust-lang/rust/pull/151426.
dbc2193d37