cmd/compile/internal/syntax: compute BranchStmt.Target statements

cmd/compile/internal/syntax: compute BranchStmt.Target statements

- Add new BranchStmt.Target field: It's the destination for break,
  continue, or goto statements.

- When parsing with CheckBranches enabled, set the BranchStmt.Target
  field. We get the information practically for free from the branch
  checker, so keep it for further use.

- Fix a couple of comments.

- This could use a test, but the new Target field is currently not
  used, and writing a test is tedious w/o a general tree visitor.
  Do it later. For now, visually verified output from syntax dump.

Change-Id: Id691d89efab514ad885e19ac9759506106579520
Reviewed-on: https://go-review.googlesource.com/40988
Reviewed-by: Matthew Dempsky <mdempsky@google.com>