Replace InsertLane format with TernaryImm8

The InsertLane format has an ordering (`value().imm().value()`) and immediate name (`"lane"`) that make it awkward to use for other instructions. This changes the ordering (`value().value().imm()`) and uses the default name (`"imm"`) throughout the codebase.

cranelift/codegen/src/verifier/mod.rs

@@ -758,7 +758,7 @@ impl<'a> Verifier<'a> {
             | Binary { .. }
             | BinaryImm { .. }
             | Ternary { .. }
-            | InsertLane { .. }
+            | TernaryImm8 { .. }
             | ExtractLane { .. }
             | Shuffle { .. }
             | IntCompare { .. }
@@ -1918,14 +1918,14 @@ impl<'a> Verifier<'a> {
                 arg,
                 ..
             }
-            | ir::InstructionData::InsertLane {
+            | ir::InstructionData::TernaryImm8 {
                 opcode: ir::instructions::Opcode::Insertlane,
-                lane,
+                imm: lane,
                 args: [arg, _],
                 ..
             } => {
                 // We must be specific about the opcodes above because other instructions are using
-                // the ExtractLane/InsertLane formats.
+                // the same formats.
                 let ty = self.func.dfg.value_type(arg);
                 if u16::from(lane) >= ty.lane_count() {
                     errors.fatal((