nir/large_constants: more careful data copying

nir/large_constants: more careful data copying

A filed of nir_variable.location may be equel to -1.
That may cause copying to invalid address of list-node,
making some internal fields corrupted.

Patch fixes segfault during freeing context due to
corrupted address of ralloc_header.destructor.

v2: copy data if var is constant (Connor Abbott)

CC: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
Fixes: b6d475356846 (nir/large_constants: De-duplicate constants)
Signed-off-by: Sergii Romantsov <sergii.romantsov@globallogic.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=111676
Reviewed-by: Connor Abbott <cwabbott0@gmail.com>