nir: add loop unroll support for wrapper loops

nir: add loop unroll support for wrapper loops

This adds support for unrolling the classic

    do {
        // ...
    } while (false)

that is used to wrap multi-line macros. GLSL IR also wraps switch
statements in a loop like this.

shader-db results IVB:

total loops in shared programs: 2515 -> 2512 (-0.12%)
loops in affected programs: 33 -> 30 (-9.09%)
helped: 3
HURT: 0

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>