simple_gvn: recognize commutative operators (#6135)

* simple_gvn: recognize commutative operators

Normalize instructions with commutative opcodes by sorting the arguments. This
means instructions like `iadd v0, v1` and `iadd v1, v0` will be considered
identical by GVN and deduplicated.

* Remove `UsubSat` and `SsubSat` from `is_commutative`

They are not actually commutative

* Remove `TODO`s

* Move InstructionData normalization into helper fn

* Add normalization of commutative instructions in the epgrah implementation

* Handle reflexive icmp/fcmps in GVN

* Change formatting of `normalize_in_place`

* suggestions from code review

cranelift/codegen/src/egraph.rs

@@ -134,6 +134,11 @@ where
     /// - Update the value-to-opt-value map, and update the eclass
     ///   union-find, if we rewrote the value to different form(s).
     pub(crate) fn insert_pure_enode(&mut self, inst: NewOrExistingInst) -> Value {
+        match inst {
+            NewOrExistingInst::New(mut data, _) => data.normalize_in_place(),
+            NewOrExistingInst::Existing(inst) => self.func.dfg.insts[inst].normalize_in_place(),
+        }
+
         // Create the external context for looking up and updating the
         // GVN map. This is necessary so that instructions themselves
         // do not have to carry all the references or data for a full