compiler: Require mir_opt_level > 1 for SingleUseConsts MIR pass

[Enselic]

To make tests/debuginfo/basic-stepping.rs pass.

Note that we must keep SingleUseConsts enabled for tests/ui-fulldeps/rustc_public/check_allocation.rs since its fn collect_consts() assumes that:

VarDebugInfo {
    name: "_false",
    [...]
    value: Place(
        _5,
    ),
}

is transformed into the following which is what SingleUseConsts does:

VarDebugInfo {
    name: "_false",
    [...]
    value: Const(
        [...]
    ),
}

Closes: #33013

CC author @celinval and reviewer @ouz-a of 0a0e7e6 as they may help determine if the fix to tests/ui-fulldeps/rustc_public/check_allocation.rs is acceptable or if some other change is needed.

[rustbot]

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

[rustbot]

r? @chenyukang

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[Enselic]

I have an idea for a better fix to check_allocation.rs.

@rustbot author