nir: Allow more than just ALU instructions in 'weak' GVN

nir: Allow more than just ALU instructions in 'weak' GVN

This removes the ALU-only restriction on the "weak" GVN introduced by
the previous commit.  This makes it slightly more aggressive, allowing
it to coalesce things like UBO loads (still within sister then/else
blocks).  This also can have surprisingly large cascading effects.

I was concerned that this might increase register pressure, but
shader-db and fossil-db show effectively no change in spills/fills,
so it seems to be fine.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/19823>