[dev.regabi] go/constant: avoid heap allocations in match

When type switching from interface{} to T, and then returning the T as
interface{} again, it's better to return the original interface{}
value. This avoids needing to heap allocate the T for
non-pointer-shaped types (i.e., int64Val, complexVal, stringVal).

Change-Id: I25c83b3f9ec9bd2ffeec5a65279b68f4fcef8a19
Reviewed-on: https://go-review.googlesource.com/c/go/+/272647


Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>