nir: Add a nop intrinsic

This intrinsic has no destination, no sources, no variables, and can be
eliminated.  In other words, it does nothing and will always get deleted by
dead code elimination.  However, it does provide a quick-and-easy way to
temporarily tag a particular location in a NIR shader.

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Cc: "12.0" <mesa-stable@lists.freedesktop.org>

src/compiler/nir/nir_intrinsics.h

@@ -41,6 +41,9 @@
 
 #define ARR(...) { __VA_ARGS__ }
 
+INTRINSIC(nop, 0, ARR(0), false, 0, 0, 0, xx, xx, xx,
+          NIR_INTRINSIC_CAN_ELIMINATE)
+
 INTRINSIC(load_var, 0, ARR(0), true, 0, 1, 0, xx, xx, xx, NIR_INTRINSIC_CAN_ELIMINATE)
 INTRINSIC(store_var, 1, ARR(0), false, 0, 1, 1, WRMASK, xx, xx, 0)
 INTRINSIC(copy_var, 0, ARR(0), false, 0, 2, 0, xx, xx, xx, 0)