[PATCH] [PR20893] Change SelectionDAG's DbgValueMap to have set semantics (NFC)

[Adrian Prantl]

Hi echristo, dblaikie, friss,

The attached patch changes the DbgValMap to have set semantics, which fixes the excessive memory allocation followed by a crash reported in PR20893. (http://llvm.org/bugs/show_bug.cgi?id=20893)
When SelectionDAG is combining two SDNodes and then merging their DbgValues via DAG.TransferDbgValues(), SelectionDAG::AddDbgValue() doesn't check whether a DbgValue is already in that SDNode's list of DbgValues and happily adds all of them. In cases like the one in the PR, this can lead to a pathological situation where — when dagcombining a long chain of instructions — the SDNode's DbgValMap is growing exponentially.

http://reviews.llvm.org/D5451

Files:
  include/llvm/CodeGen/SelectionDAG.h
  lib/CodeGen/SelectionDAG/SDNodeDbgValue.h
  lib/CodeGen/SelectionDAG/SelectionDAG.cpp
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