src/cmd/5c/reg.c:
. Added USED() attribute.

src/cmd/6c/cgen.c:
. Revised code around "REGARG" to resemble use in "8c" and
  consequently remove a warning.

src/cmd/6l/asm.c:
. Added USED() attributes.
. Removed an unnecessary assignment.

R=golang-dev
CC=golang-dev, rsc
https://golang.org/cl/4836045