cmd/6g, cmd/8g, cmd/5g: make the undefined instruction have no successors

cmd/6g, cmd/8g, cmd/5g: make the undefined instruction have no successors

The UNDEF instruction was listed in the instruction data as having the next instruction in the stream as its successor. This confused the optimizer into adding a load where it wasn't needed, in turn confusing the liveness analysis pass for GC bitmaps into thinking that the variable was live.

Fixes #7229.

LGTM=iant, rsc
R=golang-codereviews, bradfitz, iant, dave, rsc
CC=golang-codereviews
https://golang.org/cl/56910045