[TargetRegisterInfo] Introduce a helper class, BitMaskClassIterator, to

[TargetRegisterInfo] Introduce a helper class, BitMaskClassIterator, to
iterate over register class bitmask.

Thanks to this helper class, it would not require for each user of the
register classes bitmask to actually know how they are represents.
Moreover, it will make the code much easier to read.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@265730 91177308-0d34-0410-b5e6-96231b3b80d8