exp/ssa: add dedicated Panic instruction.

By avoiding the need for self-loops following calls to panic,
we reduce the number of basic blocks considerably.

R=gri
CC=golang-dev, iant
https://codereview.appspot.com/7403043
cmd/fix: delete pre-Go 1 fixes

Assume people who were going to update to Go 1 have done so.
Those with pre-Go 1 trees remaining will need to update first
to Go 1.0 (using its 'go fix') and then to Go 1.1.

Cuts the cmd/fix test time by 99% (3 seconds to 0.03 seconds).

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/7402046