spirv: Use the system value version of gl_FrontFace
spirv: Use the system value version of gl_FrontFace

SPIR-V treats it as an input but NIR wants the system value.  This
shouldn't have been too much of a surprise given that we have to do the
same conversion in the GLSL IR to NIR pass.

Signed-off-by: Jason Ekstrand <68c46a606457643eab92053c1c05574abb26f861@jlekstrand.net>
Reviewed-by: Kenneth Graunke <bd2562f754ec92342f93f61c25d731e290a2ffa8@whitecape.org>
Cc: "12.0" <59f39c0db42d4479a46b02d4d2bc11120e37bb44@lists.freedesktop.org>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler