cmd/compile: better name for BloopWalk

cmd/compile: better name for BloopWalk

Use bloop.Walk instead of BloopWalk for better naming.

Change-Id: I60fb7abf8cdab2c7641c69d1cc06c1d6bef1600b
Reviewed-on: https://go-review.googlesource.com/c/go/+/728960
Reviewed-by: Keith Randall <khr@google.com>
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
Reviewed-by: Keith Randall <khr@golang.org>