nvc0/ir: fix second tex argument after levelZero optimization

We used to pre-set a bunch of extra arguments to a texture instruction
in order to force the RA to allocate a register at the boundary of 4.
However with the levelZero optimization, which removes a LOD argument
when it's uniformly equal to zero, we undid that logic by removing an
extra argument. As a result, we could end up with insufficient alignment
on the second wide texture argument.

Instead we switch to a different method of achieving the same result.
The logic runs during the constraint analysis of the RA, and adds unset
sources as necessary right before being merged into a wide argument.

Fixes MISALIGNED_REG errors in Hitman when run with bindless textures
enabled on a GK208.

Fixes: 9145873b ("nvc0/ir: use levelZero flag when the lod is set to 0")
Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Cc: 19.0 <mesa-stable@lists.freedesktop.org>
(cherry picked from commit 5de5beed)