cranelift: Remove brz and brnz (#5630)

Remove the brz and brnz instructions, as their behavior is now redundant with brif.

cranelift/frontend/src/frontend.rs

@@ -1163,9 +1163,8 @@ mod tests {
             }
             {
                 let arg = builder.use_var(y);
-                builder.ins().brnz(arg, block3, &[]);
+                builder.ins().brif(arg, block3, &[], block2, &[]);
             }
-            builder.ins().jump(block2, &[]);
 
             builder.switch_to_block(block2);
             if !lazy_seal {