SharedTurboAssembler is shared between ia32 and x64. It contains shared
functionality between these two backends. It derives from
TurboAssemblerBase, and the backend-specific TurboAssembler derives from
SharedTurboAssembler. That way, LiftoffAssembler does not need to
change.

As an example, we move a single member function I64x2SConvertI32x4High
into SharedTurboAssembler. This function has the exact same
implementation on ia32 and x64, and is now defined in a single place.

Bug: v8:11589
Change-Id: I1c4a0628d61960e7398d458fb813561751e0614f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2773609Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73652}