nir: only override previous alu during loop analysis if supported

nir: only override previous alu during loop analysis if supported

Users of this function expect alu to be a supported comparision
if the induction variable is not NULL. Since we attempt to
override the return values if the first limit is not a const, we
must make sure we are dealing with a valid comparision before
overriding the alu instruction.

Fixes an unreachable in inverse_comparison() with the game
Assasins Creed Odyssey.

Fixes: 3235a942c16b ("nir: find induction/limit vars in iand instructions")
Acked-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=110216