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no longer valid.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/opt_dead_code_local.cpp

@@ -197,6 +197,11 @@ process_assignment(void *ctx, ir_assignment *ir, exec_list *assignments)
 	    if (entry->lhs != var)
 	       continue;
 
+            /* Skip if the assignment we're trying to eliminate isn't a plain
+             * variable deref. */
+            if (entry->ir->lhs->ir_type != ir_type_dereference_variable)
+               continue;
+
 	    int remove = entry->unused & ir->write_mask;
 	    if (debug) {
 	       printf("%s 0x%01x - 0x%01x = 0x%01x\n",