cmd/compile/internal/types2: use correct types when checking generic conversions

cmd/compile/internal/types2: use correct types when checking generic conversions

Iterate through the actual, possibly defined types of constraints
when type-checking generic conversions, not the underlying types.

For #47150.

Change-Id: Ia7af313bf46d6f6b0ad5292ff793b030b8e2d3d8
Reviewed-on: https://go-review.googlesource.com/c/go/+/357333
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>