cmd/compile: add specialized Value reset for OpCopy

This:

* Simplifies and shortens the generated code for rewrite rules.
* Shrinks cmd/compile by 86k (0.4%) and makes it easier to compile.
* Removes the stmt boundary code wrangling from Value.reset,
  in favor of doing it in the one place where it actually does some work,
  namely the writebarrier pass. (This was ascertained by inspecting the
  code for cases in which notStmtBoundary values were generated.)

Passes toolstash-check -all.

Change-Id: I25671d4c4bbd772f235195d11da090878ea2cc07
Reviewed-on: https://go-review.googlesource.com/c/go/+/221421


Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>