nir/gcm: Add global value numbering support

nir/gcm: Add global value numbering support

Unlike the current CSE pass, global value numbering is capable of detecting
common values even if one does not dominate the other.  For instance, in
you have

if (...) {
   ssa_1 = ssa_0 + 7;
   /* use ssa_1 */
} else {
   ssa_2 = ssa_0 + 7;
   /* use ssa_2 */
}

Global value numbering doesn't care about dominance relationships so it
figures out that ssa_1 and ssa_2 are the same and converts this to

if (...) {
   ssa_1 = ssa_0 + 7;
   /* use ssa_1 */
} else {
   /* use ssa_1 */
}

Obviously, we just broke SSA form which is bad.  Global code motion,
however, will repair this for us by turning this into

ssa_1 = ssa_0 + 7;
if (...) {
   /* use ssa_1 */
} else {
   /* use ssa_1 */
}

This intended to eventually mostly replace CSE.  However, conventional CSE
may still be useful because it's less of a scorched-earth approach and
doesn't require GCM.  This makes it a bit more appropriate for use as a
clean-up in a late optimization run.

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>