nir: do not assume that the result of fexp2(a) is always an integral

It's only correct when 'a' is an integral greater or equal to 0.

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>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
(cherry picked from commit 966a455b)
(conflicts resolved by Dylan Baker)