68000: fix SP postincrement/predecrement with byte operands
68000: fix SP postincrement/predecrement with byte operands

When the address register is the stack pointer and the operand size is
byte, the address is incremented/decremented by two to keep the stack
pointer aligned to a word boundary.
[M68000 Family Programmer’s Reference Manual; 2.2.4 & 2.2.5]

fixes #1709

NationalSecurityAgency/ghidra,NationalSecurityAgency/ghidra,NationalSecurityAgency/ghidra,NationalSecurityAgency/ghidra,NationalSecurityAgency/ghidra,NationalSecurityAgency/ghidra,NationalSecurityAgency/ghidra