Make demanded bits lazy

Make demanded bits lazy

The algorithm itself is still eager, but it doesn't get run until a
query function is called. This greatly reduces the compile-time impact
of requiring DemandedBits when at runtime it is not often used.

NFCI.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@249685 91177308-0d34-0410-b5e6-96231b3b80d8