add more and significantly better comments to the rest of the machineinstr

add more and significantly better comments to the rest of the machineinstr
flags that can be set.  Add predicates for the ones lacking it, and switch
some clients over to using the predicates instead of Flags directly.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@45690 91177308-0d34-0410-b5e6-96231b3b80d8