v3d: Fix the check for "is the last thrsw inside control flow"

v3d: Fix the check for "is the last thrsw inside control flow"

The execute.file check used to be good enough, until I stopped setting up
the execute mask for uniform ifs.

No known tests fixed, noticed while doing a refactor.

Fixes: 080506057310 ("v3d: Handle dynamically uniform IF statements with uniform control flow.")