Add DerivedFunction for splitting lane widths and doubling the number of lanes

Certain operations (e.g. x86_packss) will have operands with types like `NxM` but will return results with types like `(N/2)x(M*2)` (halve the lane width, double the number of lanes; maintain the same number of vector bits). This is equivalent to applying two `DerivedFunction`s to the type: `DerivedFunction::HalfWidth` then `DerivedFunction::DoubleVector`. Since there is no easy way to apply multiple `DerivedFunction`s (e.g. most of the logic is one-level deep, 1d5a678124/cranelift/codegen/meta/src/gen_inst.rs (L618-L621)), I added `DerivedFunction::SplitLanes` to do the necessary type conversion.

cranelift/codegen/src/ir/instructions.rs

@@ -518,6 +518,9 @@ enum OperandConstraint {
 
     /// This operand is `ctrlType.double_vector()`.
     DoubleVector,
+
+    /// This operand is `ctrlType.split_lanes()`.
+    SplitLanes,
 }
 
 impl OperandConstraint {
@@ -544,6 +547,11 @@ impl OperandConstraint {
                     .expect("invalid type for half_vector"),
             ),
             DoubleVector => Bound(ctrl_type.by(2).expect("invalid type for double_vector")),
+            SplitLanes => Bound(
+                ctrl_type
+                    .split_lanes()
+                    .expect("invalid type for split_lanes"),
+            ),
         }
     }
 }