[dev.typeparams] cmd/compile/internal/syntax: not all index expressions can be instantiated types

An index expression followed by an opening "{" may indicate
a composite literal but only if the index expression can be
a type. Exclude cases where the index expression cannot be
a type (e.g. s[0], a[i+j], etc.).

This leads to a better error message in code that is erroneous.

Fixes #46558.

Change-Id: Ida9291ca30683c211812dfb95abe4969f44c474f
Reviewed-on: https://go-review.googlesource.com/c/go/+/325009


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>