tcg/s390: fix ld/st with CONFIG_TCG_PASS_AREG0

tcg/s390: fix ld/st with CONFIG_TCG_PASS_AREG0

The load/store slow path has been broken in e141ab52d:
- We need to move 4 registers for store functions and 3 registers for
  load functions and not the reverse.
- According to the s390x calling convention the arguments of a function
  should be zero extended. This means that the register shift should be
  done with TCG_TYPE_I64 to ensure the higher word is correctly zero
  extended when needed.

I am aware that CONFIG_TCG_PASS_AREG0 is being removed and thus that
this patch can be improved, but doing so means it can also be applied to
the 1.1 and 1.2 stable branches.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Alexander Graf <agraf@suse.de>