glsl: Add a lowering pass to move discards out of if-statements.

This should allow lower_if_to_cond_assign to work in the presence of
discards, fixing bug #31690 and likely #31983.

NOTE: This is a candidate for the 7.9 branch.

src/glsl/SConscript

@@ -49,6 +49,7 @@ sources = [
     'loop_analysis.cpp',
     'loop_controls.cpp',
     'loop_unroll.cpp',
+    'lower_discard.cpp',
     'lower_if_to_cond_assign.cpp',
     'lower_instructions.cpp',
     'lower_jumps.cpp',