nir/lower_int64: Fix [iu]mul_high handling

nir/lower_int64: Fix [iu]mul_high handling

e551040c602d, which added a new mechanism for 64-bit imul which is more
efficient on BDW and later Intel hardware also introduced a bug where we
weren't properly walking both X and Y.  No idea how testing didn't find
this.

Fixes: e551040c602d ("nir/glsl: Add another way of doing lower_imul64 for gen8+"
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/6306
Reviewed-by: Matt Turner <mattst88@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15829>