Expand the Flags field of MCInstrDesc to 64 bits, while simultaneously

Expand the Flags field of MCInstrDesc to 64 bits, while simultaneously
shrinking the Size and NumDefs fields to offset the size growth, and
reordering the fields to preserve a good packing.

This is necessary in the short term for adding a convergent flag, and
simultaneously future-proofs us against more flags being added in the
future.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@238445 91177308-0d34-0410-b5e6-96231b3b80d8