cmd/compile/internal/types2: rename isX predicates to allX, add simple is_X (step 1 of 2)

Rename the isX predicates to allX to clearly identify that these
predicates are looking inside type parameters.

Introduce is_X as predicates that do not look
inside type parameters so we can see all call sites.
The next CL will rename them all back to isX.

Review all call sites and use correct predicate.

Replace the single helper function is with isBasic and allBasic.

Change-Id: I3430ccfc466fdedf4b58a6158f95d47b9020f7a5

Change-Id: I81116b87cf8f2e17526723c7440676d133057aca
Reviewed-on: https://go-review.googlesource.com/c/go/+/360955


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