move assert_unsafe_preconditions to its own file

These macros and functions are not intrinsics, after all.

compiler/rustc_codegen_cranelift/src/base.rs

@@ -780,7 +780,7 @@ fn codegen_stmt<'tcx>(
                         NullOp::OffsetOf(fields) => {
                             layout.offset_of_subfield(fx, fields.iter()).bytes()
                         }
-                        NullOp::UbCheck(_) => {
+                        NullOp::UbChecks => {
                             let val = fx.tcx.sess.opts.debug_assertions;
                             let val = CValue::by_val(
                                 fx.bcx.ins().iconst(types::I8, i64::try_from(val).unwrap()),