nir: fix memleak in error path

Fixes: 2cf59861 ("nir: Add partial redundancy elimination for compares")
Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
(cherry picked from commit 7659c619)