nir: remove explicit nir_intrinsic_index_flag values

These were left after a rebase and happen to make
NIR_INTRINSIC_SWIZZLE_MASK == NIR_INTRINSIC_SRC_ACCESS, which is how it
was noticed.

Fixes: 6f20643b ("nir: Allow qualifiers on copy_deref and image instructions")
Cc: Connor Abbott <cwabbott0@gmail.com>
Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
(cherry picked from commit 5d7bcac4)