StringUTF16.(get|set)Char intrinsic should use scaled operand

-XX:+StringCharIntrinsics does:

  0x00007f7cd51ab90d: shl %edx
  0x00007f7cd51ab90f: movzwl 0x10(%r10,%rdx,1),%eax

-XX:-StringCharIntrinsics does:

; <lots on Unsafe garbage before>
0x00007f3a5cf8fa13: movzwl 0x10(%r10,%r11,2),%eax

The intrinsic could use a complex operand with scale=2.