machinst x64: fix index handling of jump table;

The index should be truncated to 32 bits before being used for the jump
table entry computation.

cranelift/codegen/src/isa/x64/inst/emit.rs

@@ -1336,7 +1336,7 @@ pub(crate) fn emit(
             // We generate the following sequence:
             // ;; generated by lowering: cmp #jmp_table_size, %idx
             // jnb $default_target
-            // mov %idx, %tmp2
+            // movl %idx, %tmp2
             // lea start_of_jump_table_offset(%rip), %tmp1
             // movzlq [%tmp1, %tmp2], %tmp2
             // addq %tmp2, %tmp1
@@ -1349,7 +1349,8 @@ pub(crate) fn emit(
             };
             one_way_jmp(sink, CC::NB, *default_label); // idx unsigned >= jmp table size
 
-            let inst = Inst::gen_move(*tmp2, *idx, I64);
+            // Copy the index (and make sure to clear the high 32-bits lane of tmp2).
+            let inst = Inst::movzx_rm_r(ExtMode::LQ, RegMem::reg(*idx), *tmp2);
             inst.emit(sink, flags, state);
 
             // Load base address of jump table.