Re-land "[SCEVExpander] Use llvm data structures; NFC"

This change re-lands r289215, by reverting r289482.  The underlying
issue that caused it to be reverted has been fixed by Tim Northover in
r289496.

Original commit message for r289215:

[SCEVExpander] Use llvm data structures; NFC

Original commit message for r289482:

Revert "[SCEVExpander] Use llvm data structures; NFC"

This reverts r289215 (git SHA1 cb7b86a1).  It breaks the ubsan build
because a DenseMap that keys off of `AssertingVH<T>` will hit UB when it
tries to cast the empty and tombstone keys to `T *` (due to insufficient
alignment).

This is the relevant stack trace (thanks to Mike Aizatsky):

    #0 0x25cf100 in llvm::AssertingVH<llvm::PHINode>::getValPtr() const llvm/include/llvm/IR/ValueHandle.h:212:39
    #1 0x25cea20 in llvm::AssertingVH<llvm::PHINode>::operator=(llvm::AssertingVH<llvm::PHINode> const&) llvm/include/llvm/IR/ValueHandle.h:234:19
    #2 0x25d0092 in llvm::DenseMapBase<llvm::DenseMap<llvm::AssertingVH<llvm::PHINode>, llvm::detail::DenseSetEmpty, llvm::DenseMapInfo<llvm::AssertingVH<llvm::PHINode> >, llvm::detail::DenseSetPair<llvm::AssertingVH<llvm::PHINode> > >, llvm::AssertingVH<llvm::PHINode>, llvm::detail::DenseSetEmpty, llvm::DenseMapInfo<llvm::AssertingVH<llvm::PHINode> >, llvm::detail::DenseSetPair<llvm::AssertingVH<llvm::PHINode> > >::clear() llvm/include/llvm/ADT/DenseMap.h:113:23

llvm-svn: 289602