Support degenerate bitfields with fields of length 32 and 64

Fixes http://d.puremagic.com/issues/show_bug.cgi?id=11160 / http://d.puremagic.com/issues/show_bug.cgi?id=8474

Adds tests for:
http://d.puremagic.com/issues/show_bug.cgi?id=6686
http://d.puremagic.com/issues/show_bug.cgi?id=5942
Which seem to have been fixed over time.