Prohibit thin-air values

[lars-t-hansen]

Brian Demsky observed (in email) that our current memory model may allow values to be constructed out of thin air in the context of data races. Classical example where x and y are in shared memory:

r1 = x                          r2 = y
if (r1)                         if (r2)
  y = 1                           x = 1

Need to investigate this. If found to be the case the straightforward fix is probably to outlaw it as a side condition, as C++ has done. This is not great but at the moment may be the best we can do.

[lars-t-hansen]

The forthcoming memory model rewrite attempts to outlaw this with a rule about certain value speculations, so I'll close this and we can reopen or create new bugs if the problem is believed to persist.