Add a cranelift flag to enable/disable verbose logs for regalloc2 (#4481)

cranelift/codegen/src/machinst/compile.rs

@@ -33,7 +33,7 @@ pub fn compile<B: LowerBackend + TargetIsa>(
     let regalloc_result = {
         let _tt = timing::regalloc();
         let mut options = RegallocOptions::default();
-        options.verbose_log = log::log_enabled!(log::Level::Trace);
+        options.verbose_log = b.flags().regalloc_verbose_logs();
         regalloc2::run(&vcode, machine_env, &options)
             .map_err(|err| {
                 log::error!(