Several fixes:

(1) Applied patch from Casey to implement iterator::operator= correctly:
    it should use a pointer, not a reference.
(2) Added operators == and !=, and method all().
(3) Important bug fix: excess bits need to be ignored in operations
    like ==, count(), and all().  We do this by ensuring excess bits
    in the last bitset are always 0.

llvm-svn: 4837