nir: Perform 'weak' global value numbering in all GCM passes

nir: Perform 'weak' global value numbering in all GCM passes

Full global value numbering (GVN) can be pretty aggressive, moving
values far away from their original locations, even out of loops,
and can extend their live ranges a lot.  So we've left it disabled.

This patch introduces a weaker form of GVN: we only allow coalescing
identical values when they appear on either side of the same if/else
construct.  For now, we also only allow ALU instructions.

This allows nir_opt_gcm to clean up identical instructions appearing
on both sides of if/then/else control flow.  But it avoids aggressively
combining every other occurrence of a value in the program.

This can still have surprisingly large cascading effects, as simple
constructs are cleaned up, leading to more opportunities to do the
same clean up, up a chain of nested ifs.  It also enables greater use
of the select peephole as ifs are cleaned up.

shader-db and fossil-db results show a reduction in spills/fills on
Icelake, so it doesn't seem to be hurting register pressure.

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