[ValueTracking] Move constant range computation into ValueTracking; NFC

InstructionSimplify currently has some code to determine the constant
range of integer instructions for some simple cases. It is used to
simplify icmps.

This change moves the relevant code into ValueTracking as
llvm::computeConstantRange(), so it can also be reused for other
purposes.

In particular this is with the optimization of overflow checks in
mind (ref D59071), where constant ranges cover some cases that
known bits don't.

llvm-svn: 355781