compiler: include global variable preinit blocks in ast dumps

Dump out the blocks corresponding to variable pre-inits when
-fgo-dump-ast is in effect. Each preinit block is prefixed with a
comment indicating the variable it is initializing.

Change-Id: Id507920c566fab28ee07bc55b89d5c4cb788441d
Reviewed-on: https://go-review.googlesource.com/118636
Reviewed-by: Ian Lance Taylor <iant@golang.org>
1 file changed