Add legalization helper instructions.

The isplit_lohi instruction breaks an integer into two halves. This will
typically be used to get the two halves of an `i64` value on 32-bit
CPUs.

The iconcat_lohi is the reverse operation. It reconstructs the `i64`
from the low and high bits.

src/libcretonne/ir/instructions.rs

@@ -120,6 +120,12 @@ pub enum InstructionData {
         opcode: Opcode,
         ty: Type, // TBD: imm: Box<ImmVectorData>
     },
+    UnarySplit {
+        opcode: Opcode,
+        ty: Type,
+        second_result: Value,
+        arg: Value,
+    },
     Binary {
         opcode: Opcode,
         ty: Type,