nir: fix lowering to scratch with boolean access
Signed-off-by:Rhys Perry <pendingchaos02@gmail.com> Reviewed-by:
Jason Ekstrand <jason@jlekstrand.net> Fixes: 18ed82b0 ('nir: Add a pass for selectively lowering variables to scratch space') Part-of: <mesa/mesa!5214> (cherry picked from commit 8e2009c4)