Promoted constants lead to poor MIR from comparisons

[scottmcm]

Follow-along at https://rust.godbolt.org/z/Trzrobvo5

Consider these two functions that ought to be equivalent:

use std::cmp::Ordering;

#[no_mangle]
pub fn direct(e: Option<Ordering>) -> bool {
    e == Some(Ordering::Equal)
}

#[no_mangle]
pub fn with_let(e: Option<Ordering>) -> bool {
    let eq = Ordering::Equal;
    e == Some(eq)
}

The latter ends up with about the MIR you'd expect: it checks for Some, then for Equal

    bb0: {
        StorageLive(_2);
        _2 = discriminant(_1);
        switchInt(move _2) -> [0: bb1, 1: bb2, otherwise: bb4];
    }

    bb1: {
        _0 = const false;
        goto -> bb3;
    }

    bb2: {
        StorageLive(_3);
        _3 = discriminant(((_1 as Some).0: std::cmp::Ordering));
        _0 = Eq(copy _3, const 0_i8);
        StorageDead(_3);
        goto -> bb3;
    }

    bb3: {
        StorageDead(_2);
        return;
    }

    bb4: {
        unreachable;
    }

The former, though, ends up with way more MIR because of the promoted const:

    bb0: {
        _2 = const direct::promoted[0];
        StorageLive(_5);
        StorageLive(_4);
        StorageLive(_3);
        _3 = discriminant(_1);
        switchInt(move _3) -> [0: bb1, 1: bb2, otherwise: bb6];
    }

    bb1: {
        _4 = discriminant((*_2));
        _0 = Eq(copy _4, const 0_isize);
        goto -> bb5;
    }

    bb2: {
        _5 = discriminant((*_2));
        switchInt(move _5) -> [0: bb3, 1: bb4, otherwise: bb6];
    }

    bb3: {
        _0 = const false;
        goto -> bb5;
    }

    bb4: {
        StorageLive(_6);
        StorageLive(_7);
        _6 = discriminant(((_1 as Some).0: std::cmp::Ordering));
        _7 = discriminant((((*_2) as Some).0: std::cmp::Ordering));
        _0 = Eq(copy _6, copy _7);
        StorageDead(_7);
        StorageDead(_6);
        goto -> bb5;
    }

    bb5: {
        StorageDead(_3);
        StorageDead(_4);
        StorageDead(_5);
        return;
    }

    bb6: {
        unreachable;
    }

Apparently we can't even tell what the discriminant is of that const direct::promoted[0], so we end up emitting even LLVM IR checks on the value of the constant, which it then has to turn around and optimize away again.

Can we maybe just unilaterally un-promote simple (Copy + NoCell + Scalar/ScalarPair) constants like this?