glsl: Fix array indexing when constant folding built-in functions.

Mesa constant-folds built-in functions by using a miniature GLSL
interpreter (see
ir_function_signature::constant_expression_evaluate_expression_list()).
This interpreter had a bug in its handling of array indexing, which
caused expressions like "m[i][j]" (where m is a matrix) to be handled
incorrectly.  Specifically, it incorrectly treated j as indexing into
the whole matrix (rather than indexing just into the vector m[i]); as
a result the offset computed for m[i] was lost and m[i][j] was treated
as m[j][0].

Fixes piglit tests inverse-mat[234].{vert,frag}.

NOTE: This is a candidate for the 9.1 and 9.0 branches.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=57436

src/glsl/ir_constant_expression.cpp

@@ -1398,7 +1398,7 @@ ir_dereference_array::constant_referenced(struct hash_table *variable_context,
       return;
    }
 
-   const glsl_type *vt = substore->type;
+   const glsl_type *vt = array->type;
    if (vt->is_array()) {
       store = substore->get_array_element(index);
       offset = 0;