exp/ssa: add Instruction.Operands and Value.Referrers methods.

Operands returns the SSA values used by an instruction.
Referrers returns the SSA instructions that use a value, for
some values.  These will be used for SSA renaming, to follow.

R=iant, gri
CC=golang-dev
https://codereview.appspot.com/7312090
net: change server_test.go so we could see failure messages

R=golang-dev, dave
CC=golang-dev
https://codereview.appspot.com/7323051