[ConstantRange] Add abs() support

Add support for abs() to ConstantRange. This will allow to handle
SPF_ABS select flavor in LVI and will also come in handy as a
primitive for the srem implementation.

The implementation is slightly tricky, because a) abs of signed min
is signed min and b) sign-wrapped ranges may have an abs() that is
smaller than a full range, so we need to explicitly handle them.

Differential Revision: https://reviews.llvm.org/D61084

llvm-svn: 359321