cmd/compile: add logging for large (>= 128 byte) copies

For 1.15, unless someone really wants it in 1.14.

A performance-sensitive user thought this would be useful,
though "large" was not well-defined.  If 128 is large,
there are 139 static instances of "large" copies in the compiler
itself.

Includes test.

Change-Id: I81f20c62da59d37072429f3a22c1809e6fb2946d
Reviewed-on: https://go-review.googlesource.com/c/go/+/205066


Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>