x64: Shrink Inst from 72 to 48 bytes (#4514)

https://github.com/bytecodealliance/wasmtime/pull/4514

cranelift/codegen/src/isa/x64/lower/isle.rs

@@ -30,10 +30,12 @@ use crate::{
         VCodeConstantData,
     },
 };
+use smallvec::SmallVec;
 use std::boxed::Box;
 use std::convert::TryFrom;
 
 type BoxCallInfo = Box<CallInfo>;
+type BoxVecMachLabel = Box<SmallVec<[MachLabel; 4]>>;
 
 pub struct SinkableLoad {
     inst: Inst,