[dev.typeparams] cmd/compile/internal/types2: factor out constraint satisfaction check

This is a simple move of a block of inlined code into a function
to make instantiation more manageable and easier to understand.
There is no change in functionality or behavior.

Change-Id: I46e7a9ea03527731e1f0219b3402eb03949627c5
Reviewed-on: https://go-review.googlesource.com/c/go/+/322070


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