PPC Simd regs are already using separate set of register banks
on ppc, more details can be found here:
https://crrev.com/c/2718472

Here we are making use of this CL https://crrev.com/c/3005768
(fcd3ef48) and fully separating Simd regs during register allocation.

Member function `toSimd()` is also introduced which will be used
to cast FP regs to Simd regs in liftoff.

Change-Id: Ic5551fb04f37de7fc9501a2f1aba8fb44f622d95
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3755213
Commit-Queue: Milad Farazmand <mfarazma@redhat.com>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81707}