cmd/compile/internal/types2: review of stmt.go

The changes between (equivalent, and reviewed) go/types/stmt.go
and stmt.go can be seen by comparing patchset 1 and 2. The actual
changes are removing the "// UNREVIEWED" marker, and minor adjustments
to get the code slightly closer to go/types/stmt.go.

The primary differences compared to go/types are:
- use of syntax rather than go/ast package, with significant
  differences in the representation of switch and select statements,
  range clauses of for statements, and inc/dec statements.
- no reporting of error codes
- use or error_ for collecting addition error information

Change-Id: I4409f62ecafd0653e4c8ef087c2580d8f0544efc
Reviewed-on: https://go-review.googlesource.com/c/go/+/305576


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