Collapse empty blocks

[b-paul]

proc @main(a:bv64) -> (c:bv64)
[
    block %inputs [ goto(%a); ];
    block %a [ goto(%b, %c); ];
    block %b [ goto(%c); ];
    block %c [ var b:bv64 := a:bv64; goto(%d); ];
    block %d [ goto(%e); ];
    block %e ( var c_1:bv64 := phi(%d -> b:bv64) ) [
        var c:bv64 := c_1:
        goto(%f);
    ];
    block %f [ return; ];
];
prog entry @main;

becomes

proc @main(a:bv64)  -> (c:bv64) {  }
[
   block %c [ var b:bv64 := a:bv64; goto (%e); ];
   block %e ( var c_1:bv64 := phi(%c -> b:bv64) ) [
     var c:bv64 := c_1:bv64;
     goto (%f);
   ];
   block %f [ return; ]
];
prog entry @main;