cmd/compile: refactor preserveStmt

cmd/compile: refactor preserveStmt

By breaking it into smaller functions with duplicated logic removed.

Change-Id: I710414e8d13d91c19fb4bf4da1a31b00ee88ccee
Reviewed-on: https://go-review.googlesource.com/c/go/+/728961
Reviewed-by: Junyang Shao <shaojunyang@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>