[Comb] Fold mux if both arms are same constant

If both arms of a `comb.mux` evaluate to the same constant, fold the mux
to that same constant. Right now this is only done if the SSA value is
identical. Some constant propagation algorithms will call `fold` to
propagate lattice constants, which isn't covered by just comparing SSA
values.

Short: also check adaptor's true and false value for folding, not just
the op's true and false value.

lib/Dialect/Comb/CombFolds.cpp

@@ -1989,6 +1989,9 @@ OpFoldResult MuxOp::fold(FoldAdaptor adaptor) {
   // mux (c, b, b) -> b
   if (getTrueValue() == getFalseValue())
     return getTrueValue();
+  if (auto tv = adaptor.getTrueValue())
+    if (tv == adaptor.getFalseValue())
+      return tv;
 
   // mux(0, a, b) -> b
   // mux(1, a, b) -> a