nir: Fix printing of ~0 .locations.

I kept wondering what "429" meant in variable declarations, when it was
just a truncated ~0 snprintf.

Reviewed-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3423>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3423>

src/compiler/nir/nir_print.c

@@ -562,9 +562,13 @@ print_var_decl(nir_variable *var, print_state *state)
       }
 
       if (!loc) {
+         if (var->data.location == ~0) {
+            loc = "~0";
+         } else {
             snprintf(buf, sizeof(buf), "%u", var->data.location);
             loc = buf;
          }
+      }
 
       /* For shader I/O vars that have been split to components or packed,
        * print the fractional location within the input/output.