[dev.typeparams] cmd/compile: add base.Assertf{,At} functions

We have almost 200 uses of the "assert" helper functions in noder and
typecheck. Clearly the tiny bit of extra convenience of writing a
one-line assertion rather than an if+panic is helpful, so we might as
well add functions for this to base itself so that it's easier to
write more informative error messages.

Change-Id: I06e2db2f0455af063937b25a53ca42f9413cf496
Reviewed-on: https://go-review.googlesource.com/c/go/+/336050


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>