Pack MachineInstr fields better.

This shrinks MachineInstr to 64 bytes (from 72).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@171813 91177308-0d34-0410-b5e6-96231b3b80d8
Merge "Pack MachineInstr fields better." into jb42