[IPSCCP] Move the IPSCCP run function under the IPO directory.

The LLVMipo library no longer depends on the Scalar component.
The shared functions between IPSCCP and SCCP have been moved
under Utils, in the SCCPSolver.

This is preliminary work for D126455, in order to break a cyclic
dependency between LLVM libraries.

Differential Revision: https://reviews.llvm.org/D138654