nir: fix if condition propagation for alu use

nir: fix if condition propagation for alu use

We need to update the cursor before we check if the alu use is
dominated by the if condition. Previously we were checking if
the current location of the alu instruction was dominated by
the if condition which would miss some optimisation opportunities.

Fixes: a3b4cb34589e ("nir/opt_if: Rework condition propagation")
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>