optimize enum of #[repr(int)] enums with non-overlapping discriminants to just use inner discriminant

[programmerjake]

currently this layout optimization is not done:

// could be size 2 since A and B's discriminants are in the same spot with non-overlapping contiguous values, but currently isn't
enum Top {
    A(A),
    B(B),
}

#[repr(u8)]
enum A {
    A0(u8) = 0,
    A1(u8) = 1,
}

#[repr(u8)]
enum B {
    B2(u8) = 2,
    B3(u8) = 3,
}

previously mentioned: #46213 (comment)

[programmerjake]

@rustbot label +A-codegen +A-layout +C-enhancement +T-compiler

[the8472]

Dup of #119055

[workingjubilee]

closing as duplicate.