glsl: fix compiling warning from gcc 4.7

glsl: fix compiling warning from gcc 4.7

ir_validate.cpp: In member function ‘virtual ir_visitor_status ir_validate::visit_leave(ir_swizzle*)’:
ir_validate.cpp:458:66: warning: narrowing conversion of ‘ir->ir_swizzle::mask.ir_swizzle_mask::x’ from ‘unsigned int’ to ‘int’ inside { } is ill-formed in C++11 [-Wnarrowing]
ir_validate.cpp:458:66: warning: narrowing conversion of ‘ir->ir_swizzle::mask.ir_swizzle_mask::y’ from ‘unsigned int’ to ‘int’ inside { } is ill-formed in C++11 [-Wnarrowing]
ir_validate.cpp:458:66: warning: narrowing conversion of ‘ir->ir_swizzle::mask.ir_swizzle_mask::z’ from ‘unsigned int’ to ‘int’ inside { } is ill-formed in C++11 [-Wnarrowing]
ir_validate.cpp:458:66: warning: narrowing conversion of ‘ir->ir_swizzle::mask.ir_swizzle_mask::w’ from ‘unsigned int’ to ‘int’ inside { } is ill-formed in C++11 [-Wnarrowing]

Signed-off-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Brian Paul <brianp@vmware.com>