SelectionDAG: Remove deleted nodes from legalized set to avoid clash with newly created nodes

Summary:
During DAG legalization loop in SelectionDAG::Legalize(),
bookkeeping of the SDNodes that were already legalized is implemented
with SmallPtrSet (LegalizedNodes). This kind of set stores only pointers
to objects, not the objects themselves. Unfortunately, if SDNode is
deleted during legalization for some reason, LegalizedNodes set is not
informed about this fact. This wouldn’t be so bad, if SelectionDAG wouldn’t reuse
space deallocated after deletion of unused nodes, for creation of new
ones. Because of this, new nodes, created during legalization often can
have pointers identical to ones that have been previously legalized,
added to the LegalizedNodes set, and deleted afterwards. This in turn
causes, that newly created nodes, sharing the same pointer as deleted
old ones, are present in LegalizedNodes *already at the moment of
creation*, so we never call Legalize on them.
The fix facilitates the fact, that DAG notifies listeners about each
modification. I have registered DAGNodeDeletedListener inside
SelectionDAG::Legalize, with a callback function that removes any
pointer of any deleted SDNode from the LegalizedNodes set. With this
modification, LegalizeNodes set does not contain pointers to nodes that
were deleted, so newly created nodes can always be inserted to it, even
if they share pointers with old deleted nodes.

Patch by pawel.szczerbuk@intel.com

The issue this patch addresses causes failures in an out-of-tree target,
and i was not able to create a reproducer for an in-tree target, hence
there is no test-case.

Reviewers: delena, spatel, RKSimon, hfinkel, davide, qcolombet

Reviewed By: delena

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D33891

llvm-svn: 305084