[dev.power64] 6g: don't create variables for indirect addresses

Previously, mkvar treated, for example, 0(AX) the same as AX.
As a result, a move to an indirect address would be marked as
*setting* the register, rather than just using it, resulting
in unnecessary register moves.  Fix this by not producing
variables for indirect addresses.

LGTM=rsc
R=rsc, dave
CC=golang-codereviews
https://codereview.appspot.com/164610043