intel/nir: Call alu_to_scalar one last time before out-of-ssa

intel/nir: Call alu_to_scalar one last time before out-of-ssa

A few of our very late passes can end up generating vectors accidentally
so we need to get rid of them.  The only known case of this is the ffma
peephole which generates fneg and fabs as vectors.  Currently, they're
not a problem because they get turned into fmov which the back-end
compiler knows how to handle as a vector.  That's about to change.

Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>
Acked-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>