[dev.typeparams] cmd/compile/internal/types2: adjust errors in branch checking code, fix a bug

The types2.Config.IgnoreBranches flag mistakenly excluded a
set of label-unrelated branch checks. After fixing this and
also adjusting some error messages to match the existing
compiler errors, more errorcheck tests pass now with the -G
option.

Renamed IngnoreBranches to IgnoreLabels since its controlling
label checks, not all branch statement (such as continue, etc)
checks.

Change-Id: I0819f56eb132ce76c9a9628d8942af756691065a
Reviewed-on: https://go-review.googlesource.com/c/go/+/285652


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