cranelift: Port most of simple_preopt.rs over to the peepmatic DSL

This ports all of the identity, no-op, simplification, and canonicalization
related optimizations over from being hand-coded to the `peepmatic` DSL. This
does not handle the branch-to-branch optimizations or most of the
divide-by-constant optimizations.

cranelift/codegen/src/ir/dfg.rs

@@ -234,11 +234,7 @@ impl DataFlowGraph {
 
     /// Get the type of a value.
     pub fn value_type(&self, v: Value) -> Type {
-        match self.values[v] {
-            ValueData::Inst { ty, .. }
-            | ValueData::Param { ty, .. }
-            | ValueData::Alias { ty, .. } => ty,
-        }
+        self.values[v].ty()
     }
 
     /// Get the definition of a value.
@@ -383,9 +379,14 @@ pub enum ValueDef {
 impl ValueDef {
     /// Unwrap the instruction where the value was defined, or panic.
     pub fn unwrap_inst(&self) -> Inst {
+        self.inst().expect("Value is not an instruction result")
+    }
+
+    /// Get the instruction where the value was defined, if any.
+    pub fn inst(&self) -> Option<Inst> {
         match *self {
-            Self::Result(inst, _) => inst,
-            _ => panic!("Value is not an instruction result"),
+            Self::Result(inst, _) => Some(inst),
+            _ => None,
         }
     }
 
@@ -428,6 +429,16 @@ enum ValueData {
     Alias { ty: Type, original: Value },
 }
 
+impl ValueData {
+    fn ty(&self) -> Type {
+        match *self {
+            ValueData::Inst { ty, .. }
+            | ValueData::Param { ty, .. }
+            | ValueData::Alias { ty, .. } => ty,
+        }
+    }
+}
+
 /// Instructions.
 ///
 impl DataFlowGraph {