glsl: Use array deref for access to vector components

We've assumed that we could lower per-component vector access from

  vec[i] = scalar

to

  vec = ir_triop_vector_insert(vec, scalar, i)

but with SSBOs (and compute shader SLM and tesselation outputs) this is
no longer valid. If a vector is "externally visible", multiple threads
can write independent components simultaneously. With lowering to
ir_triop_vector_insert, each thread read the entire vector, changes one
component, then writes out the entire vector. This is racy.

Instead of generating a ir_binop_vector_extract when we see v[i], we
generate ir_dereference_array. We then add a lowering pass to lower the
ir_dereference_array to ir_binop_vector_extract for rvalues and for to
vector_insert for lvalues in a separate lowering pass.

The resulting IR is the same as before, but we now have a window between
ast->ir conversion and the lowering pass where v[i] appears in the IR as
an array deref. This lets us run lowering passes that lower the vector
access to I/O (eg for SSBO load/store) before we lower the per-component
access to full vector writes.

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Signed-off-by: Kristian Høgsberg Kristensen <krh@bitplanet.net>

src/glsl/ast_array_index.cpp

@@ -319,10 +319,9 @@ _mesa_ast_array_index_to_hir(void *mem_ctx,
     * expression.
     */
    if (array->type->is_array()
-       || array->type->is_matrix()) {
+       || array->type->is_matrix()
+       || array->type->is_vector()) {
       return new(mem_ctx) ir_dereference_array(array, idx);
-   } else if (array->type->is_vector()) {
-      return new(mem_ctx) ir_expression(ir_binop_vector_extract, array, idx);
    } else if (array->type->is_error()) {
       return array;
    } else {