The 'ForRegs' method is not needed for clarity of the code, it's pretty
clear what to construct a register list from. Hence turn the static
{LiftoffRegList::ForRegs} method into a constructor. This makes the code
more concise.

Also, turn the for loop into a C++17 folding expression, which might
generate better code because the loop will automatically be unrolled.

R=thibaudm@chromium.org

Bug: v8:12425
Change-Id: Ic4446f23022db5f17420303d45b9c15fa5daff60
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3572041Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79815}