nir/lower_int64: Enable lowering of 64-bit float to 64-bit integer conversions.

The existing code for this appears to work okay for conversions
involving 64-bit floats, relax the assert and enable the lowering
path.  This fixes 64-bit float to 64-bit integer integer conversions
on devices that have native support for 64-bit floats but lack 64-bit
integer support, like Intel MTL hardware.

Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Part-of: <!19128>