nir: Don't forget if-uses in new nir_opt_dead_cf liveness check
Commit 08bfd710. (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: 08bfd710 nir/dead_cf: Stop relying on liveness analysis