spirv: Move to compiler/

While it does rely on NIR, it's not really part of the NIR core.  At the
moment, it still builds as part of libnir but that can be changed later if
desired.

src/intel/vulkan/anv_pipeline.c

@@ -31,7 +31,7 @@
 #include "anv_private.h"
 #include "brw_nir.h"
 #include "anv_nir.h"
-#include "nir/spirv/nir_spirv.h"
+#include "spirv/nir_spirv.h"
 
 /* Needed for SWIZZLE macros */
 #include "program/prog_instruction.h"