[dev.power64] 9g: proginfo fixes
[dev.power64] 9g: proginfo fixes

For D_OREG addresses, store the used registers in regindex
instead of reguse because they're really part of addressing.

Add implicit register use/set for DUFFZERO/DUFFCOPY.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://codereview.appspot.com/174050044

webfd/go-zh,webfd/go-zh,bryanxu/go-zh,bryanxu/go-zh,d0f/go-zh,bryanxu/go-zh,d0f/go-zh,d0f/go-zh,webfd/go-zh,d0f/go-zh,d0f/go-zh,d0f/go-zh,webfd/go-zh,bryanxu/go-zh,bryanxu/go-zh,bryanxu/go-zh,webfd/go-zh,webfd/go-zh,d0f/go-zh,bryanxu/go-zh,d0f/go-zh,bryanxu/go-zh,webfd/go-zh,webfd/go-zh