Merge rip-relative loading and call/jump into one instruction for
OFF_HEAP_TARGET call/jump. For example,

  REX.W movq r10,[rip+#disp]
  call r10

turns into:

  call [rip+#disp]

Change-Id: I17e115d054b4b352bdaf8eba2e6ac4054bbedaca
Reviewed-on: https://chromium-review.googlesource.com/1172152
Commit-Queue: Shiyu Zhang <shiyu.zhang@intel.com>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55150}