[PATCH] Make InstCombine aware of TargetTransformInfo when optimize extension

[Xuetian Weng]

Hi dberlin, chandlerc,

In NVPTX64, 64-bit register is simulated by two 32-bit register which
makes it more expensive on 64-bit operations.

In this patch we compute the extra cost of extending the expression
tree for zext and sext by exploiting TargetTransformInfo and make sure
the transform will not bring more costs.

http://reviews.llvm.org/D10750

Files:
  lib/Transforms/InstCombine/InstCombineCasts.cpp
  lib/Transforms/InstCombine/InstCombineInternal.h
  lib/Transforms/InstCombine/InstructionCombining.cpp
  test/Transforms/InstCombine/NVPTX/lit.local.cfg
  test/Transforms/InstCombine/NVPTX/no-widen-expensive.ll

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