nir: Don't forget if-uses in new nir_opt_dead_cf liveness check

nir: Don't forget if-uses in new nir_opt_dead_cf liveness check

Commit 08bfd710a25c14df5f690cce9604617536d7c560. (nir/dead_cf: Stop
relying on liveness analysis) introduced a new check that iterated
through a SSA def's uses, to see if it's used.  But it only checked
normal uses, and not uses which are part of an 'if' condition.  This
led to it thinking more nodes were dead than possible.

Fixes Piglit's variable-indexing/tcs-output-array-float-index-wr test
(and related tests) with the out-of-tree Iris driver.

Fixes: 08bfd710a25 nir/dead_cf: Stop relying on liveness analysis
Reviewed-by: Connor Abbott <cwabbott0@gmail.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>