[ValueTracking][InstSimplify] Move abs handling into computeConstantRange(); NFC

This is preparation for D59506. The InstructionSimplify abs handling
is moved into computeConstantRange(), which is the general place for
such calculations. This is NFC and doesn't affect the existing tests
in test/Transforms/InstSimplify/icmp-abs-nabs.ll.

Differential Revision: https://reviews.llvm.org/D59511

llvm-svn: 356409