glsl_types: fix build break with intel/msvc compiler

glsl_types: fix build break with intel/msvc compiler

The VECN() macro was taking advantage of a GCC specific feature that is
not available on lesser compilers, mostly for the purposes of avoiding a
macro that encoded a return statement.

But as suggested by Ian, we could just have the macro produce the entire
method body and avoid the need for this.  So let's do that instead.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=105740
Fixes: f407edf3407396379e16b0be74b8d3b85d2ad7f0
Cc: Emil Velikov <emil.velikov@collabora.com>
Cc: Timothy Arceri <tarceri@itsqueeze.com>
Cc: Roland Scheidegger <sroland@vmware.com>
Cc: Ian Romanick <idr@freedesktop.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>
Acked-by: Timothy Arceri <tarceri@itsqueeze.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>