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"),
+            ),
         }
     }
 }

cranelift/codegen/src/ir/types.rs

@@ -284,7 +284,7 @@ impl Type {
 
     /// Split the lane width in half and double the number of lanes to maintain the same bit-width.
     ///
-    /// If this is a scalar type of n bits, it produces a SIMD vector type of (n/2)x2.
+    /// If this is a scalar type of `n` bits, it produces a SIMD vector type of `(n/2)x2`.
     pub fn split_lanes(self) -> Option<Self> {
         match self.half_width() {
             Some(half_width) => half_width.by(2),
@@ -292,6 +292,17 @@ impl Type {
         }
     }
 
+    /// Merge lanes to half the number of lanes and double the lane width to maintain the same
+    /// bit-width.
+    ///
+    /// If this is a scalar type, it will return `None`.
+    pub fn merge_lanes(self) -> Option<Self> {
+        match self.double_width() {
+            Some(double_width) => double_width.half_vector(),
+            None => None,
+        }
+    }
+
     /// Index of this type, for use with hash tables etc.
     pub fn index(self) -> usize {
         usize::from(self.0)