The "to" field was the penultimate argument to outgcode,
instead of the last argument, which swapped the third and
fourth operands.  The argument order was correct in a.y, so
just swap the meaning of the arguments in outgcode.  This
hadn't come up because we hadn't used these more obscure
operations in any hand-written assembly until now.

LGTM=rsc, dave
R=rsc, dave
CC=golang-codereviews
https://golang.org/cl/160690043