[APInt] Add isSubsetOf method that can check if one APInt is a subset of...

[APInt] Add isSubsetOf method that can check if one APInt is a subset of another without creating temporary APInts

This question comes up in many places in SimplifyDemandedBits. This makes it easy to ask without allocating additional temporary APInts.

The BitVector class provides a similar functionality through its (IMHO badly named) test(const BitVector&) method. Though its output polarity is reversed.

I've provided one example use case in this patch. I plan to do more as a follow up.

Differential Revision: https://reviews.llvm.org/D32258

llvm-svn: 300851