Change MachineOperand::OpKind from unsigned char to a bitfield. NFC.

This causes OpKind and all the bitfields after it to use 32-bit load/stores instead of i24's for the existing bitfields.

Bugs will be filed to track whether clang and llvm could have generated the 32-bit operations in the front-end or optimizer.

Reviewed by Rafael.

llvm-svn: 235528