The DoubleToI stub is no longer called outside of TurboFan, and always in the
same way:

  - The parameter is on top of the stack.
  - The stub is always called in a slow path.
  - It truncates.

Therefore, we can simplify it to only support this case and remove dead
code.

On top of this, since the stub is always considered to be on a slow path for all
backends, this patch takes the opportunity to remove the `skip_fastpath`
optimisation. This would generate a stub which does not handle all inputs,
assuming that the backend already handled some of the inputs in a fast
path. Removing this allows the stub to have the same behaviour on all targets.

On Arm, this patch reworks the stub a little. We could use ip instead of saving
and restoring a register on the stack. Also, comments would mention that we
assume the exponent to be greater than 31 when the it can be 30 or higher. As
done for Arm64, let's check this at runtime in debug mode.

On Arm64, we can also implement the stub without pushing and poping off the
stack. It needs 2 general purpose and a double scratch registers which we have
reserved already (ip0, ip1 and d30). This removes the need to check that the
stack pointer is always 16-bytes aligned.

Finally, this also fixes a potential bug on Arm64, in the
`GetAllocatableRegisterThatIsNotOneOf` method which is now removed. We were
picking an allocatable double register when we meant to pick a general one.

Bug: v8:6644
Change-Id: I88d4597f377c9fc05432d5922a0d7129b6d19b47
Reviewed-on: https://chromium-review.googlesource.com/720963Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Pierre Langlois <pierre.langlois@arm.com>
Cr-Commit-Position: refs/heads/master@{#48671}