bifrost: Constant fold after lower_explicit_io

nir_lower_explicit_io generates mul+add chains even for constants.  One
round of constant folding should get rid of these.  This fixes all of
the dEQP-VK.glsl.conversions.* tests on panvk.

GoGoGoGo'd-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>

Part-of: <!15349>