Uniform (bool) while conditions do not compile in a kernel

[byeongjee]

A while whose condition is a plain bool does not compile inside a kernel:

#[kernel]
fn f(n: i32, x: Varying<f32>) -> Varying<f32> {
    let mut acc = x;
    let mut i = 0;
    while i < n {
        acc = acc + 1.0;
        i += 1;
    }
    acc
}

Two errors:

error[E0308]: mismatched types
 |     while i < n {
 |     ----- ^^^ expected `Mask<i32, N>`, found `bool`

error[E0277]: cannot assign to a value of type `i32` under execution context `VMask<N>`
 |         i += 1;

The first is the condition slot of Exec::enter_loop. The second is more
fundamental: rewrite.rs:390 rebinds __exec to the loop mask inside the body,
so the loop counter is a uniform write under a varying mask — deliberately not
implemented, since every lane would race on one location.

Both workarounds already exist and are used by the benchmark kernels:

for i in 0..n { ... }
loop { if !c { break; } ... }