st/mesa: Lower vars to ssa and constant prop before gl_nir_lower_buffers

st/mesa: Lower vars to ssa and constant prop before gl_nir_lower_buffers

The gl_nir_lower_buffers pass relies on recognizing the same literal
constants as the GLSL compiler so that constant buffer array indices
are constant in nir as well.  Without this, get_block_array_index()
would see

  vec1 32 ssa_723 = deref_var &const_temp@1 (function_temp int)
  vec1 32 ssa_724 = load_const (0x00000001 /* 0.000000 */)
  ...
  vec1 32 ssa_5 = deref_var &const_temp@1 (function_temp int)
  vec1 32 ssa_6 = intrinsic load_deref (ssa_5) (0) /* access=0 */
  vec1 32 ssa_7 = deref_var &blockB (ssbo BlockB[1])
  vec1 32 ssa_8 = deref_array &(*ssa_7)[ssa_6] (ssbo BlockB) /* &blockB[ssa_6] */

instead of a literal 1, and ultimately generate the block name
BlockB[0].  That used to work, since we before the previous commits
we'd compact the block binding points and names. Thus, there would
always be a BlockB[0].

Now, if an entry in a block array isn't used, we don't generate that
block name, which means that if entry 0 isn't used BlockB[0] isn't
present and then get_block_array_index() fails to find the block.

In most cases we would have dealt with this in the call to
st_nir_opts() in st_nir_link_shaders(), but in the num_shaders == 1
case (for example, compute) we would call gl_nir_lower_buffers()
before we lowered GLSL constants.  Move that corner case up next to
where we call st_nir_link_shaders() so we call st_nir_opts() at the
same point in the flow for all shaders.

Fixes: dEQP-GLES31.functional.ssbo.layout.random.all_per_block_buffers.18
Signed-off-by: Kristian H. Kristensen <hoegsberg@google.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>