Reduce arm64 Inst enum size

This reduces the size of the Inst enum from 112 bytes to 48 bytes.

Using DHAT on a regex-rs.wasm benchmark, `valgrind --tool=dhat clif-util compile --target aarch64`

The total number of allocated bytes, drops by around 170 MB.
At t-gmax drops by 3 MB.

Using `perf stat clif-util compile --target aarch64`, the instructions count dropped by 0.6%. Cache misses dropped by 6%. Cycles dropped by 2.3%.

cranelift/codegen/src/isa/aarch64/lower_inst.rs

@@ -2132,8 +2132,8 @@ pub(crate) fn lower_branch<C: LowerCtx<I = Inst>>(
                     ridx,
                     rtmp1,
                     rtmp2,
-                    targets: jt_targets,
-                    targets_for_term,
+                    targets: jt_targets.into_boxed_slice(),
+                    targets_for_term: targets_for_term.into_boxed_slice(),
                 });
             }