Add a cctest to verify correct treatment of functions which return SIMD.

This exercises the MoveToReturnRegisters logic, where we need to add a
new case for fp pairs. FinishCall also needs to handle fp pairs.

Small cleanup to rename needs_reg_pair to needs_gp_reg_pair to be clear,
and add a new helper needs_fp_reg_pair.

Drive-by fix for cctest to assert that the tests are compiled with
Liftoff.

Bug: v8:9909
Change-Id: I0cd7a1a90e97372ea85e7668f2298d4fa2d76f4f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2006021
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65848}