nir/opcodes: Fix constant-folding of bitfield_insert

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=104119


CC: <mesa-stable@lists.freedesktop.org>
CC: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
(cherry picked from commit 947470d1)