go spec: constant divisors must not be zero
go spec: constant divisors must not be zero

Both gc and gccgo always checked this for constant
expressions but the spec only mentions run-time
exceptions.

This CL also requires that constant divisors
must not be zero in non-constant integer expressions:
This is consistent with the spirit of the most
recent changes and it is consistent with constant
expressions. We don't want to specify the effect for
non-integer expressions (f/0.0 where f is a float or
complex number) because there the result f/g is not
further specified if a non-constant g is 0.

R=r, rsc, iant, ken, andybalholm, iant
CC=golang-dev
http://codereview.appspot.com/6710045

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang