cmd/compile: sanitize before/after expansion OpSelectN references

In expand_calls, OpSelectN occurs both before and after the rewriting.
Attempting to rewrite a post-expansion OpSelectN is bad.
(The only ones rewritten in place are the ones returning mem;
others are synthesized to replace other selection chains with
register references.)

Updates #40724.
Updates #44816#issuecomment-815258897.

Change-Id: I7b6022cfb47f808d3ce6cc796c067245f36047f3
Reviewed-on: https://go-review.googlesource.com/c/go/+/308309


Trust: David Chase <drchase@google.com>
Run-TryBot: David Chase <drchase@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>