glsl: Fix chained assignments of vector channels.

Simple shaders such as:

    void splat(vec2 v, float f) {
        v[0] = v[1] = f;
    }

failed to compile with the following error:
error: value of type vec2 cannot be assigned to variable of type float

First, we would process v[1] = f, and transform:
LHS: (expression float vector_extract (var_ref v) (constant int (1)))
RHS: (var_ref f)
into:
LHS: (var_ref v)
RHS: (expression vec2 vector_insert (var_ref v) (constant int (1))
                 (var_ref f))

Note that the LHS type is now vec2, not a float.  This is surprising,
but not the real problem.

After emitting assignments, this ultimately becomes:
(declare (temporary) vec2 assignment_tmp)
(assign (xy)
  (var_ref assignment_tmp)
  (expression vec2 vector_insert (var_ref v) (constant int (1))
              (var_ref f)))
  (assign (xy) (var_ref v) (var_ref assignment_tmp))

We would then return (var_ref assignment_tmp) as the rvalue, which has
the wrong type---it should be float, but is instead a vec2.

To fix this, we simply return (vector_extract (var_ref assignment_temp)
<the appropriate channel>) to pull out the desired float value.

Fixes Piglit's chained-assignment-with-vector-constant-index.vert and
chained-assignment-with-vector-dynamic-index.vert tests.

Cc: mesa-stable@lists.freedesktop.org
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=74026
Reported-by: Dan Ginsburg <dang@valvesoftware.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>

src/glsl/ast_to_hir.cpp

@@ -740,6 +740,7 @@ do_assignment(exec_list *instructions, struct _mesa_glsl_parse_state *state,
 {
    void *ctx = state;
    bool error_emitted = (lhs->type->is_error() || rhs->type->is_error());
+   ir_rvalue *extract_channel = NULL;
 
    /* If the assignment LHS comes back as an ir_binop_vector_extract
     * expression, move it to the RHS as an ir_triop_vector_insert.
@@ -755,11 +756,23 @@ do_assignment(exec_list *instructions, struct _mesa_glsl_parse_state *state,
          if (new_rhs == NULL) {
             return lhs;
          } else {
+            /* This converts:
+             * - LHS: (expression float vector_extract <vec> <channel>)
+             * - RHS: <scalar>
+             * into:
+             * - LHS: <vec>
+             * - RHS: (expression vec2 vector_insert <vec> <channel> <scalar>)
+             *
+             * The LHS type is now a vector instead of a scalar.  Since GLSL
+             * allows assignments to be used as rvalues, we need to re-extract
+             * the channel from assignment_temp when returning the rvalue.
+             */
+            extract_channel = lhs_expr->operands[1];
             rhs = new(ctx) ir_expression(ir_triop_vector_insert,
                                          lhs_expr->operands[0]->type,
                                          lhs_expr->operands[0],
                                          new_rhs,
-                                         lhs_expr->operands[1]);
+                                         extract_channel);
             lhs = lhs_expr->operands[0]->clone(ctx, NULL);
          }
       }
@@ -856,6 +869,11 @@ do_assignment(exec_list *instructions, struct _mesa_glsl_parse_state *state,
    if (!error_emitted)
       instructions->push_tail(new(ctx) ir_assignment(lhs, deref_var));
 
+   if (extract_channel) {
+      return new(ctx) ir_expression(ir_binop_vector_extract,
+                                    new(ctx) ir_dereference_variable(var),
+                                    extract_channel->clone(ctx, NULL));
+   }
    return new(ctx) ir_dereference_variable(var);
 }