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

This reverts commit 42c2dc40.

This broke some buildbots:

 undefined reference to `llvm::createBitTrackingDCEPass()'
 undefined reference to `llvm::createAlignmentFromAssumptionsPass()'
 undefined reference to `llvm::createLoopUnrollPass(int, bool, bool, int, int, int, int, int, int)'
 undefined reference to `llvm::createLICMPass(unsigned int, unsigned int, bool)'
 undefined reference to `llvm::createWarnMissedTransformationsPass()'
 undefined reference to `llvm::createAlignmentFromAssumptionsPass()'
 undefined reference to `llvm::createCallSiteSplittingPass()'
 undefined reference to `llvm::createCFGSimplificationPass(llvm::SimplifyCFGOptions, std::function<bool (llvm::Function const&)>)'
 undefined reference to `llvm::createFloat2IntPass()'
 undefined reference to `llvm::createLowerConstantIntrinsicsPass()'
 undefined reference to `llvm::createLoopRotatePass(int, bool)'
 undefined reference to `llvm::createLoopDistributePass()'
 undefined reference to `llvm::createLoopSinkPass()'
 undefined reference to `llvm::createInstSimplifyLegacyPass()'
 undefined reference to `llvm::createDivRemPairsPass()'
 undefined reference to `llvm::createCFGSimplificationPass(llvm::SimplifyCFGOptions, std::function<bool (llvm::Function const&)>)'
 undefined reference to `llvm::SetLicmMssaOptCap'
 undefined reference to `llvm::SetLicmMssaNoAccForPromotionCap'
 undefined reference to `llvm::ForgetSCEVInLoopUnroll'