glsl: Initialize lower_vector_visitor::dont_lower_swz.

Fixes "Uninitialized scalar field" defect reported by Coverity.

Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/glsl/lower_vector.cpp

@@ -35,7 +35,7 @@ namespace {
 
 class lower_vector_visitor : public ir_rvalue_visitor {
 public:
-   lower_vector_visitor() : progress(false)
+   lower_vector_visitor() : dont_lower_swz(false), progress(false)
    {
       /* empty */
    }