cmd/compile: mark unused values as invalid to prevent problems in expandCalls

Leftover values that have been replaced can cause problems in later
passes (within expandCalls).  For example, a struct select that
itself yields a struct will have a problematic rewrite, if the chance
is presented.

Updates #40724.

Change-Id: I1b445c47c301c3705f7fc0a9d39f1f5c84f4e190
Reviewed-on: https://go-review.googlesource.com/c/go/+/306869


Trust: David Chase <drchase@google.com>
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>