cranelift: Remove brz and brnz (#5630)

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

cranelift/frontend/src/lib.rs

@@ -50,8 +50,7 @@
 //!    jump block1
 //! block1:
 //!    z = z + y;
-//!    brnz y, block3;
-//!    jump block2
+//!    brif y, block3, block2
 //! block2:
 //!    z = z - x;
 //!    return y
@@ -122,9 +121,8 @@
 //!     }
 //!     {
 //!         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);
 //!     builder.seal_block(block2);