[dev.power64] 5g,6g,8g,9g: debug prints for regopt pass 6 and paint2

Theses were very helpful in understanding the regions and
register selection when porting regopt to 9g.  Add them to the
other compilers (and improve 9g's successor debug print).

LGTM=rsc
R=rsc
CC=golang-codereviews
https://codereview.appspot.com/174130043