nir: do not assume that the result of fexp2(a) is an integral
It's only correct when 'a' is a positive integral.
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=111493 Fixes: 5544b2cb ("nir/algebraic: Use value range analysis to eliminate useless unary ops") Signed-off-by: Samuel Pitoiset samuel.pitoiset@gmail.com