cmd/compile/internal/noder: reduce clutter a bit (cosmetic changes)

- introduce index alias
- inline the two short tables in stmt.go (removes a TODO)
- move assert out of stencil.go and remove that file
  (we can always re-introduce it)

Also, replace two if's with a simpler switch.

Change-Id: I25c3104164574999dd9826dee6166dd8a8488908
Reviewed-on: https://go-review.googlesource.com/c/go/+/607236


Reviewed-by: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Tim King <taking@google.com>