nir/validate: Validated dests after sources

nir/validate: Validated dests after sources

Previously, if someone accidentally made an instruction that refers to its
own SSA destination, the validator wouldn't catch it.  The reason for this
is that it validated the destination too early and, by the time it got to
the source, the destination SSA value was already added to the set of seen
SSA values so it would assume that it came from some previous instruction.
By moving destination validation to be after source validation, the SSA
value is not in the list of seen values and the validator will catch
self-referential instructions.

Reviewed-by: Connor Abbott <cwabbott0@gmail.com>