[PATCH] Use rsqrt (X86) to speed up reciprocal square root calcs (PR20900)

[Sanjay Patel]

This version of the patch brings the 2-constant NR builder into DAGCombiner and adds a target-specified boolean to select whether we should use the 2-constant version or the 1-constant version.

We can add a command-line override for the NR selector in a subsequent patch.

http://reviews.llvm.org/D5658

Files:
  include/llvm/Target/TargetLowering.h
  lib/CodeGen/SelectionDAG/DAGCombiner.cpp
  lib/Target/PowerPC/PPCISelLowering.cpp
  lib/Target/PowerPC/PPCISelLowering.h
  lib/Target/X86/X86.td
  lib/Target/X86/X86ISelLowering.cpp
  lib/Target/X86/X86ISelLowering.h
  lib/Target/X86/X86Subtarget.cpp
  lib/Target/X86/X86Subtarget.h
  test/CodeGen/X86/sqrt-fastmath.ll
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