glsl_to_tgsi: Assert that the _mesa_sysval_to_semantic mapping is correct

glsl_to_tgsi: Assert that the _mesa_sysval_to_semantic mapping is correct

Future patches will rearrange the values in gl_system_value, and I want
to catch errors.  Designated initializers would make all of this
unnecessary.

v2: Don't use STATIC_ASSERT.  Not only does it not work, but GCC doesn't
tell you that it's not going to work.  Thanks for nothing!

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>