[dev.typeparams] cmd/compile: fix bug with types2.Instantiate with interface type param

types2.subst has an assertion that check is non-nil, but which breaks
Instantiate() with an interface type param (used when re-importing
instatiated type to types2). But this check was added when Instantiate()
was added, and things seem to work fine when the assertion is removed.

Fixes test/typeparam/mdempsky/7.go.

Change-Id: I4980f0b202a0b310a3c91a7a87f97576f54911de
Reviewed-on: https://go-review.googlesource.com/c/go/+/333155


Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
Trust: Dan Scales <danscales@google.com>