glsl: Fix g++ initializer order warning

glsl: Fix g++ initializer order warning

Fixes this warning:
In file included from ../../../src/compiler/glsl/ir.cpp:25:0:
../../../src/compiler/glsl/ir.h: In constructor 'ir_swizzle::ir_swizzle(ir_rvalue*, ir_swizzle_mask)':
../../../src/compiler/glsl/ir.h:1955:20: warning: 'ir_swizzle::mask' will be initialized after [-Wreorder]
    ir_swizzle_mask mask;
                    ^
../../../src/compiler/glsl/ir.h:1954:15: warning:   'ir_rvalue* ir_swizzle::val' [-Wreorder]
    ir_rvalue *val;
               ^
../../../src/compiler/glsl/ir.cpp:1592:1: warning:   when initialized here [-Wreorder]
 ir_swizzle::ir_swizzle(ir_rvalue *val, ir_swizzle_mask mask)
 ^

Reviewed-by: Matt Turner <mattst88@gmail.com>