glsl: Use the generated constant expression code

Immediately previous to this patch,

    diff -wud src/glsl/ir_constant_expression.cpp \
              src/glsl/ir_expression_operation_constant.h

should be "minimal."

v3: With much help from José Fonseca, fix the SCons build.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
diff --git a/src/compiler/SConscript.glsl b/src/compiler/SConscript.glsl
index b1b4169..d9878a0 100644
--- a/src/compiler/SConscript.glsl
+++ b/src/compiler/SConscript.glsl
@@ -124,7 +124,12 @@
     source = [],
     command = python_cmd + ' $SCRIPT enum > $TARGET'
 )
-
+env.CodeGenerate(
+    target = 'glsl/ir_expression_operation_constant.h',
+    script = 'glsl/ir_expression_operation.py',
+    source = [],
+    command = python_cmd + ' $SCRIPT constant > $TARGET'
+)
 env.CodeGenerate(
     target = 'glsl/ir_expression_operation_strings.h',
     script = 'glsl/ir_expression_operation.py',