radv: Add RADV_DEBUG=validatevas for address validation in nir

The option creates a buffer where each bit stores whether the
corresponding 4096 byte memory section has been allocated. The helper
radv_build_is_valid_va allows for querying the validity of addresses
inside a nir shader which can be useful for debugging.

Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/34392>

docs/envvars.rst

@@ -1481,6 +1481,8 @@ RADV driver environment variables
       Dump shader disassembly for selected shader stages.
    ``bvh4``
       Use bvh4 encoding on GPUs that support bvh8 encoding.
+   ``validatevas``
+      Enable tracking of VA ranges for radv_build_is_valid_va.
 
 .. envvar:: RADV_FORCE_FAMILY