[Mips] Fix inst `sc` disassemble assert when configured -mattr=+ptr64 (#158253)
When mips disassembler process `DecodeMem`, does not consider
Mips::SC64, leading to `Operands.size() = 3`.
And the right `Size` value is 4, when printMemOperand would occur
asserts: `assert(idx < size());`
Fix #157508.