We already don't convert constants out of SSA, and in our backend we'd
like to have only one way of saying something is still in SSA.
The one tricky part about this is that we may now leave some undef
instructions around if they aren't part of a phi-web, so we have to be
more careful about deleting them.
v2: rename and flip meaning of flag (Jason)
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>
2b1a1d8b12