glsl: Allow ir_assignment() constructor to not specify condition.

We almost never want to specify a condition, and when we do we're
already thinking about it (because we're writing a lowering pass
generating the condition), so a default argument should make the code
more pleasant to read.

NOTE: This is a candidate for the 7.11 branch (we want to be able to
cherry-pick future code).
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
(cherry picked from commit e617a53a74)

src/glsl/ir.h

@@ -682,7 +682,7 @@ public:
 
 class ir_assignment : public ir_instruction {
 public:
-   ir_assignment(ir_rvalue *lhs, ir_rvalue *rhs, ir_rvalue *condition);
+   ir_assignment(ir_rvalue *lhs, ir_rvalue *rhs, ir_rvalue *condition = NULL);
 
    /**
     * Construct an assignment with an explicit write mask