Handling of &(T{}) assumed that the parser would not introduce ()'s.
Also: Better comments around handling of OPAREN syntax tree optimization.
Fixes#13261.
Change-Id: Ifc5047a0448f5e7d74cd42f6608b87dcc9c2f2fb
Reviewed-on: https://go-review.googlesource.com/17040
Reviewed-by: Chris Manghane <cmang@golang.org>