insert_mov_immed_arch sets wrong size for reg target on x64

I hit this in Dr. Memory x64 on Windows which uses instrlist_insert_mov_instr_addr(). On this bb:

before instrumentation:
TAG  0x000000013fac1288
 +0    L3                      48 83 ec 28          sub    $0x0000000000000028 %rsp -> %rsp
 +4    L3                      e8 13 27 00 00       call   $0x000000013fac39a4 %rsp -> %rsp 0xfffffff8(%rsp)[8byte]
END 0x000000013fac1288

The return address for DrM's invocation of the slowpath is sign-extended rather than having the proper zero top bits:

 +67   L3                      48 83 ec 28          sub    $0x0000000000000028 %rsp -> %rsp
 +71   m4 @0x00000000bfc51648  48 b9 8c 12 ac 3f 01 mov    $0x000000013fac128c -> %rcx
                               00 00 00
 +81   m4 @0x00000000bfc520b0  48 c7 c0 30 e3 be bf mov    @0x00000000bfc51040[4byte] -> %rax
 +88   m4 @0x00000000bfc51360  e9 9a 38 27 00       jmp    $0x00000000bfe61bcf
 +93   m4 @0x00000000bfc51040                       <label>
 +93   m4 @0x00000000bfc512e8  65 48 8b 0c 25 b0 15 mov    %gs:0x000015b0[8byte] -> %rcx
                               00 00

=>

  0x00000000bfb71334  48 83 ec 28          sub    $0x0000000000000028 %rsp -> %rsp
  0x00000000bfb71338  48 b9 8c 12 ac 3f 01 mov    $0x000000013fac128c -> %rcx
                      00 00 00
  0x00000000bfb71342  48 c7 c0 4e 13 b7 bf mov    $0xffffffffbfb7134e -> %rax
  0x00000000bfb71349  e9 81 08 2f 00       jmp    $0x00000000bfe61bcf
  0x00000000bfb7134e  65 48 8b 0c 25 b0 15 mov    %gs:0x000015b0[8byte] -> %rcx
                      00 00