parse attrs if a loop's parent hir node is a stmt

[usamoi]

fix this case:

// unroll doesn't work
#[unsafe(no_mangle)]
fn f(n: usize) -> usize {
    let mut sum = 0;
    let mut i = 0;
    #[unroll(4)]
    loop { if i < n { sum += i; i += 1; } else { break; } }
    sum
}

// unroll works
#[unsafe(no_mangle)]
fn g(n: usize) {
    let mut i = 0;
    #[unroll(4)]
    loop { if i < n { i += 1; } else { break; } }
}

cc #156874

r? saethlin

[saethlin]

Suggested change

	// HIR for a `loop` statement is a bit different, make sure we still apply the metadata in that case.

View changes since the review

[saethlin]

@JonathanBrouwer Is this the right way to fix this? I really don't know enough about HIR to be confident that this is the right way to handle things or if this is hack on a hack.

View changes since the review