mesa: Add partial constant propagation pass for Mesa IR

This cleans up some code generated by the IR-to-Mesa pass for i915.
In particular, some shaders involving arrays of constant matrices
result in really bad code.

v2: Silence several warnings from merging the gl_constant_value work.
Fix DP[23] folding.  Add support for a bunch more opcodes that appear
in piglit runs on i915.

Reviewed-by: Eric Anholt <eric@anholt.net>
diff --git a/src/mesa/program/prog_optimize.h b/src/mesa/program/prog_optimize.h
index 463f5fc..9854fb7 100644
--- a/src/mesa/program/prog_optimize.h
+++ b/src/mesa/program/prog_optimize.h
@@ -44,4 +44,7 @@
 extern void
 _mesa_optimize_program(struct gl_context *ctx, struct gl_program *program);
 
+extern GLboolean
+_mesa_constant_fold(struct gl_program *prog);
+
 #endif