cmd/compile,runtime: skip zero'ing order array for select statements

cmd/compile,runtime: skip zero'ing order array for select statements

The order array was zero initialized by the compiler, but ends up being
overwritten by the runtime anyway.

So let the runtime takes full responsibility for initializing, save us
one instruction per select.

Fixes #40399

Change-Id: Iec1eca27ad7180d4fcb3cc9ef97348206b7fe6b8
Reviewed-on: https://go-review.googlesource.com/c/go/+/251517
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>