cranelift: Remove brz and brnz (#5630)

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

cranelift/codegen/src/isa/s390x/inst/unwind/systemv.rs

@@ -194,8 +194,7 @@ mod tests {
 
         let mut pos = FuncCursor::new(&mut func);
         pos.insert_block(block0);
-        pos.ins().brnz(v0, block2, &[]);
-        pos.ins().jump(block1, &[]);
+        pos.ins().brif(v0, block2, &[], block1, &[]);
 
         pos.insert_block(block1);
         pos.ins().return_(&[]);