Broke tests on 386.

««« original CL description
6l/8l: emit correct opcodes to F(SUB|DIV)R?D.

When the destination was not F0, 6l and 8l swapped FSUBD/FSUBRD and
FDIVD/FDIVRD.

R=golang-dev, dave, rsc
CC=golang-dev
https://golang.org/cl/6498092
»»»

R=golang-dev
CC=golang-dev
https://golang.org/cl/6492100