We recieved a report that split SP adjustment was not performing optimally when
compressed instructions were available. This patch more closely matches the
`FirstSPAdjustAmount` to the available instructions, so that loading and storing
the callee-saved registers is done with compressed instructions on architectures
that support them.
This patch also contains a fix to the type of `SecondSPAdjustAmount` to
correctly track and check the sign of this adjustment.