The explanation about VARDEF/VARKILL is from when liveness analysis
was performed on Progs. Now that it's performed on SSA, it should
reference their corresponding SSA ops (OpVarDef/OpVarKill) instead.

Change-Id: Icc4385b52768f6987cda162824b75340aee0b223
Reviewed-on: https://go-review.googlesource.com/76313Reviewed-by: Robert Griesemer <gri@golang.org>