[PATCH] move IR-level optimization flags into their own struct

[Sanjay Patel]

Hi echristo, hfinkel, kariddi, andreadb,

This is a preliminary step to using the IR-level floating-point fast-math-flags in the SDAG (D8900).

In this patch, we introduce the optimization flags as their own struct. As noted in the TODO comment, we should eventually share this data between the IR passes and the backend.

We also switch the existing nsw / nuw / exact bit functionality of the BinaryWithFlagsSDNode class to use the new struct. 

The tradeoff is that instead of using the free but limited space of SDNode's SubclassData, we add a data member to the subclass. This means we don't have to repeat all of the get/set methods per flag, but we're potentially adding size to all nodes of this subclass. 

In practice on 64-bit systems (measured on Linux and MacOS X), there is no size difference between an SDNode and BinaryWithFlagsSDNode after this change: they're both 80 bytes. This means that we had at least one free byte to play with due to struct alignment.

http://reviews.llvm.org/D9325

Files:
  include/llvm/CodeGen/SelectionDAGNodes.h
  lib/CodeGen/SelectionDAG/DAGCombiner.cpp
  lib/CodeGen/SelectionDAG/SelectionDAG.cpp
  lib/Target/X86/X86ISelLowering.cpp

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