Extract codegen into macro-assembler functions for reuse in Liftoff.

Some minor tweaks in I32x4TruncSatF64x2SZero and I32x4TruncSatF64x2UZero
to check dst and src overlap and move to scratch/dst accordingly. In
TurboFan we can set these restrictions in the instruction-selector, but
not in Liftoff. This doesn't make TurboFan codegen any worse, since
those restrictions are still in place.

Bug: v8:11265
Change-Id: I48f354c5ff86809bb3ddc38eca6dc8990b9b7d61
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2683208
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Deepti Gandluri <gdeepti@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72641}