Shrink MachineOperand from 40 to 32 bytes on 64-bit hosts.

Pull an unsigned out of the Contents union such that it has the same size as two
pointers and no padding.

Arrange members such that the Contents union and all pointers can be 8-byte
aligned without padding.

This speeds up code generation by 0.8% on a 64-bit host. 32-bit hosts should be
unaffected.

llvm-svn: 116857