[dev.typeparams] cmd/compile/internal/types2: limit termlist lengths

At the moment, operations on termlists are O(n^2).
This is fine for normal-sized unions, but overlong
termlist lenghts will lead to excessive type checking
times.

Limit the length of termlists to avoid "compilations
that don't finish".

Change-Id: I39a7fc61b01c9db06faeb49a0e014b1ede532710
Reviewed-on: https://go-review.googlesource.com/c/go/+/340254


Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>