When visiting the AST to add counters, there are special cases in which
the code calls cuts the walking short by returning nil. In some cases
certain nodes are ignored, e.g. Init and Cond inside IfStmt.

The fix is to explicitly walk all the children nodes (not only
Body and Else) when cutting the current walk. Similar approach
was taken with SwitchStmt and TypeSwitchStmt.

While the existing test code doesn't handle different counters in the
same line, the generated HTML report does it correctly (because it takes
column into account).

The previous behavior caused lines in function literals to not be
tracked when those literals were inside Init or Cond of an IfStmt for
example.

Fixes #14039.

Change-Id: Iad591363330843ad833bd79a0388d709c8d0c8aa
Reviewed-on: https://go-review.googlesource.com/19775Reviewed-by: Rob Pike <r@golang.org>