nir: Fix i64tof32 lowering

nir: Fix i64tof32 lowering

The round-to-nearest-even implementation found in lower_2f() is incorrect
for any value having a significand that is not directly representable
and whose non-representable part lies between 1 and half the minimum
representable value. In this case, the significand is rounded up instead
of being rounded down.

Fixes: 936c58c8fcce ("nir: Extend nir_lower_int64() to support i2f/f2i lowering")
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Tested-by: Jesse Natalie <jenatali@microsoft.com>
Acked-by: Matt Turner <mattst88@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6290>