glsl: Completely initialize value member in ir_constant constructor.

The
ir_constant::ir_constant(const struct glsl_type *type, exec_list *value_list)
did not completely initialize the entire value member.

Fixes piglit glsl-fs-sampler-numbering-2 valgrind uninitialized value
error in softpipe and llvmpipe.

src/glsl/ir.cpp

@@ -409,6 +409,9 @@ ir_constant::ir_constant(const struct glsl_type *type, exec_list *value_list)
       return;
    }
 
+   for (unsigned i = 0; i < 16; i++) {
+      this->value.u[i] = 0;
+   }
 
    ir_constant *value = (ir_constant *) (value_list->head);