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/inst/emit.rs

@@ -1303,7 +1303,7 @@ impl<O: MachSectionOutput> MachInstEmit<O> for Inst {
                 };
                 inst.emit(sink, flags, state);
                 // Emit jump table (table of 32-bit offsets).
-                for target in targets {
+                for target in targets.iter() {
                     let off = target.as_offset_words() * 4;
                     let off = i32::try_from(off).unwrap();
                     // cast i32 to u32 (two's-complement)