As they're now equal, simplify some TF opcodes by restoring jssp from csp on
stack pointer switch. Also, remove some direct references to jssp.

Bug: v8:6644
Change-Id: I20ee54fc0d536809a0aa72def43337f83cc91685
Reviewed-on: https://chromium-review.googlesource.com/857457Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Martyn Capewell <martyn.capewell@arm.com>
Cr-Commit-Position: refs/heads/master@{#50462}