go/types,types2: fix panic in reverse type inference when -lang<go1.18

go/types,types2: fix panic in reverse type inference when -lang<go1.18

Due to reverse type inference, we may not have an index expression when
type-checking a function instantiation. Fix a panic when the index expr
is nil.

Fixes #59639

Change-Id: Ib5de5e49cdb7b339653e4fb775bf5c5fdb3c6907
Reviewed-on: https://go-review.googlesource.com/c/go/+/484757
Reviewed-by: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Robert Findley <rfindley@google.com>