[CodeGen] Update for scalable MemoryType in MMO (#70452)

Remove getSizeOrUnknown call when MachineMemOperand is created.  For Scalable
TypeSize, the MemoryType created becomes a scalable_vector.

2 MMOs that have scalable memory access can then use the updated BasicAA that
understands scalable LocationSize.

Original Patch by Harvin Iriawan
Co-authored-by: David Green <david.green@arm.com>