cmd/compile/internal/types2: clean up panics in instantiation

Clean up a few issues related to panicking during invalid instantiation.
 - Panic early in instantiateLazy when check == nil and verify == true.
   Otherwise, we would panic at check.later.
 - Always panic when check == nil and verify == true, even if targs is
   of incorrect length. This is more consistent behavior.
 - Lift the check for len(posList) <= len(targs) out of
   Checker.instantiate. This is the only reason why posList is passed to
   that function, and doing this allows us to eliminate posList from
   instance. At this point instance is close to being unnecessary, so
   update a TODO to this effect.

Change-Id: Id5f44cbb1a5897aef10ce2a573aa78acd7ae4026
Reviewed-on: https://go-review.googlesource.com/c/go/+/341862


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