nv50/ir: fix integer mul lowering for u32 x u32 -> high u32

nv50/ir: fix integer mul lowering for u32 x u32 -> high u32

UNION appears to expect that all of its sources are conditionally
defined. Otherwise it inserts an unpredicated mov instruction which
overwrites the desired result. This fixes tests that use UMUL_HI, and
much less directly, unsigned integer division by a constant, which uses
this functionality in a peephole pass.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Cc: "10.1 10.2" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Ben Skeggs <bskeggs@redhat.com>