Add DerivedFunction for doubling lane widths and halving the number of lanes (i.e. merging)

Certain operations (e.g. widening) will have operands with types like `NxM` but will return results with types like `(N*2)x(M/2)` (double the lane width, halve the number of lanes; maintain the same number of vector bits). This is equivalent to applying two `DerivedFunction`s to the type: `DerivedFunction::DoubleWidth` then `DerivedFunction::HalfVector`. 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::MergeLanes` to do the necessary type conversion.

cranelift/codegen/src/ir/instructions.rs

@@ -584,6 +584,9 @@ enum OperandConstraint {
 
     /// This operand is `ctrlType.split_lanes()`.
     SplitLanes,
+
+    /// This operand is `ctrlType.merge_lanes()`.
+    MergeLanes,
 }
 
 impl OperandConstraint {
@@ -615,6 +618,11 @@ impl OperandConstraint {
                     .split_lanes()
                     .expect("invalid type for split_lanes"),
             ),
+            MergeLanes => Bound(
+                ctrl_type
+                    .merge_lanes()
+                    .expect("invalid type for merge_lanes"),
+            ),
         }
     }
 }