[PATCH] LLVM intrinsic for invariants

[hfinkel at anl.gov]

This adds the basic invariant intrinsic, noop lowering, and some basic properties:

 - llvm.invariant(undef) and llvm.invariant(true) are dead
 - llvm.invariant(false) is unreachable (this directly corresponds to the documented behavior of MSVC's __assume(0))

The intrinsic is tagged as writing arbitrarily, in order to maintain control dependencies. BasicAA has been updated, however, to return NoModRef for any particular location-based query so that we don't unnecessarily block code motion.

http://reviews.llvm.org/D178

Files:
  docs/LangRef.rst
  include/llvm/IR/Intrinsics.td
  lib/Analysis/BasicAliasAnalysis.cpp
  lib/Analysis/TargetTransformInfo.cpp
  lib/CodeGen/BasicTargetTransformInfo.cpp
  lib/CodeGen/IntrinsicLowering.cpp
  lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
  lib/Transforms/Utils/Local.cpp
  test/Analysis/BasicAA/invariant.ll
  test/CodeGen/Generic/invariant.ll
  test/Transforms/InstSimplify/invariant.ll
  test/Transforms/SimplifyCFG/invariant.ll
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