[SCCPSolver] Move helper functions inside SCCPSolver (NFC).

This patch moves a couple of helper functions from the global llvm::
namespace into the SCCPSolver class. This reduces the need for separate
SCCPSolver arguments and also limits the scope of those functions that
have quite generic names.

(The remaining isConstant and isOverdefined should ideally be removed)

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D142370