glsl: Add an optimization pass to simplify discards.

NOTE: This is a candidate for the 7.9 branch.
diff --git a/src/glsl/SConscript b/src/glsl/SConscript
index fd22f66..16f02e9 100644
--- a/src/glsl/SConscript
+++ b/src/glsl/SConscript
@@ -66,6 +66,7 @@
     'opt_dead_code.cpp',
     'opt_dead_code_local.cpp',
     'opt_dead_functions.cpp',
+    'opt_discard_simplification.cpp',
     'opt_function_inlining.cpp',
     'opt_if_simplification.cpp',
     'opt_noop_swizzle.cpp',