glsl: fix compiler global temp collisions

glsl ir creates these temps to copy global initialiser values for
example. To avoid collisions during linking due to 2 shaders in the same
stage having temps with the same name we make sure to define these as
function variables not shader globals. This will put the temps into the
global instructions wrapper created in 7c5b21c032.

Fixes: cbfc225e2b ("glsl: switch to a full nir based linker")
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/12136

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/32202>
(cherry picked from commit e34357015c)

.pick_status.json

@@ -1244,7 +1244,7 @@
         "description": "glsl: fix compiler global temp collisions",
         "nominated": true,
         "nomination_type": 2,
-        "resolution": 0,
+        "resolution": 1,
         "main_sha": null,
         "because_sha": "cbfc225e2bda2c8627a4580fa3a9b63bfb7133e0",
         "notes": null

src/compiler/glsl/glsl_to_nir.cpp

@@ -481,11 +481,14 @@ nir_visitor::visit(ir_variable *ir)
 
    switch(ir->data.mode) {
    case ir_var_auto:
-   case ir_var_temporary:
-      if (is_global)
+      if (is_global) {
          var->data.mode = nir_var_shader_temp;
-      else
-         var->data.mode = nir_var_function_temp;
+         break;
+      }
+
+      FALLTHROUGH;
+   case ir_var_temporary:
+      var->data.mode = nir_var_function_temp;
       break;
 
    case ir_var_function_in: