cmd/compile: put constant value on node inside parentheses

That's where the unified IR writer expects it.

Fixes #73476

Change-Id: Ic22bd8dee5be5991e6d126ae3f6eccb2acdc0b19
Reviewed-on: https://go-review.googlesource.com/c/go/+/667415


Reviewed-by: Junyang Shao <shaojunyang@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Keith Randall <khr@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Keith Randall <khr@google.com>