In the getBitsSet function, don't optimize for a common case that is

already covered by getLowBitsSet (i.e. when loBits==0). Consequently, remove
the default value for loBits and reorder the arguments to the more natural
loBits, hiBits order. This makes it more clear that this function is for bit
groups in the middle of the bit width and not towards one end or the other.

llvm-svn: 35312