nir: Initialize lower_flrp_progress everywhere

I don't know why I thought NIR_PASS always set the progress variable.
Derp.

Fixes: d41cdef2 ("nir: Use the flrp lowering pass instead of nir_opt_algebraic")
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Coverity CID: 1444996
Coverity CID: 1444995
Coverity CID: 1444994
Coverity CID: 1444993
Coverity CID: 1444991
Coverity CID: 1444989
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