Autos must be kept if their address reaches the control value of a
block. We didn't see this before because it is rare for an auto's
address to reach a control value without also reaching a phi or
being written to memory. We can probably optimize away the
comparisons that lead to this scenario since autos cannot alias
with pointers from elsewhere, however for now we take the
conservative approach and just ensure the auto is properly
initialised if its address reaches a control value.

Fixes #26407.

Change-Id: I02265793f010a9e001c3e1a5397c290c6769d4de
Reviewed-on: https://go-review.googlesource.com/124335Reviewed-by: David Chase <drchase@google.com>