nir/opt_intrinsics: Rework progress

nir/opt_intrinsics: Rework progress

This commit fixes two issues:  First, we were returning false regardless
of whether or not the function made progress.  Second, we were calling
nir_metadata_preserve far more often than needed; we only need to call
it once per impl.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>