cmd/compile/internal/types2: differently named types are not assignable
When checking assignability, a value of a named type (incl. a type parameter)
can never be assigned to a variable of a differently named type. Specifically,
if the types are two differently named type parameters, then values of one are
never assignable to variables of the other.
This CL clarifies the assignabiliy rules and simplifies the implementation.
Fixes #49242.
Change-Id: Id72a2c9bed5cdb726855e7a707137db1009e7953
Reviewed-on: https://go-review.googlesource.com/c/go/+/360274
Trust: Robert Griesemer <gri@golang.org> Reviewed-by: Robert Findley <rfindley@google.com>