glsl: Rename various ir_* files to lower_* and opt_*.

This helps distinguish between lowering passes, optimization passes, and
other compiler code.
diff --git a/src/glsl/SConscript b/src/glsl/SConscript
index c183e20..f9cc823 100644
--- a/src/glsl/SConscript
+++ b/src/glsl/SConscript
@@ -27,54 +27,54 @@
     'glsl_types.cpp',
     'glsl_symbol_table.cpp',
     'hir_field_selection.cpp',
-    'ir_algebraic.cpp',
     'ir_basic_block.cpp',
     'ir_clone.cpp',
-    'ir_constant_expression.cpp',
-    'ir_constant_folding.cpp',
-    'ir_constant_propagation.cpp',
-    'ir_constant_variable.cpp',
-    'ir_copy_propagation.cpp',
     'ir.cpp',
-    'ir_dead_code.cpp',
-    'ir_dead_code_local.cpp',
-    'ir_dead_functions.cpp',
-    'ir_div_to_mul_rcp.cpp',
-    'ir_explog_to_explog2.cpp',
     'ir_expression_flattening.cpp',
     'ir_function_can_inline.cpp',
     'ir_function.cpp',
-    'ir_function_inlining.cpp',
     'ir_hierarchical_visitor.cpp',
     'ir_hv_accept.cpp',
-    'ir_if_simplification.cpp',
-    'ir_if_to_cond_assign.cpp',
     'ir_import_prototypes.cpp',
-    'ir_lower_jumps.cpp',
-    'ir_mat_op_to_vec.cpp',
-    'ir_mod_to_fract.cpp',
-    'ir_noop_swizzle.cpp',
     'ir_print_visitor.cpp',
     'ir_reader.cpp',
     'ir_rvalue_visitor.cpp',
     'ir_set_program_inouts.cpp',
-    'ir_structure_splitting.cpp',
-    'ir_sub_to_add_neg.cpp',
-    'ir_swizzle_swizzle.cpp',
-    'ir_tree_grafting.cpp',
     'ir_validate.cpp',
     'ir_variable.cpp',
     'ir_variable_refcount.cpp',
-    'ir_vec_index_to_cond_assign.cpp',
-    'ir_vec_index_to_swizzle.cpp',
     'linker.cpp',
     'link_functions.cpp',
     'loop_analysis.cpp',
     'loop_controls.cpp',
     'loop_unroll.cpp',
+    'lower_div_to_mul_rcp.cpp',
+    'lower_explog_to_explog2.cpp',
+    'lower_if_to_cond_assign.cpp',
+    'lower_jumps.cpp',
+    'lower_mat_op_to_vec.cpp',
+    'lower_mod_to_fract.cpp',
     'lower_noise.cpp',
+    'lower_sub_to_add_neg.cpp',
     'lower_variable_index_to_cond_assign.cpp',
+    'lower_vec_index_to_cond_assign.cpp',
+    'lower_vec_index_to_swizzle.cpp',
+    'opt_algebraic.cpp',
+    'opt_constant_folding.cpp',
+    'opt_noop_swizzle.cpp',
     'opt_redundant_jumps.cpp',
+    'opt_constant_expression.cpp',
+    'opt_constant_propagation.cpp',
+    'opt_constant_variable.cpp',
+    'opt_copy_propagation.cpp',
+    'opt_dead_code.cpp',
+    'opt_dead_code_local.cpp',
+    'opt_dead_functions.cpp',
+    'opt_function_inlining.cpp',
+    'opt_if_simplification.cpp',
+    'opt_structure_splitting.cpp',
+    'opt_swizzle_swizzle.cpp',
+    'opt_tree_grafting.cpp',
     's_expression.cpp',
 ]