[PATCH] [SCEV] Extract and strengthen isHighCostExpansion.

[Sanjoy Das]

Hi atrick,

  1. extract `isHighCostExpansion` from IndVarSimplify into
     SCEVExpander. This part is NFC.

  2. teach `isHighCostExpansion` to consider divisions by power-of-two
     constants as cheap.

A test case is added for (2).  (2) is needed for a new user of
`isHighCostExpansion` that will be added in a subsequent change.

http://reviews.llvm.org/D8993

Files:
  include/llvm/Analysis/ScalarEvolutionExpander.h
  lib/Analysis/ScalarEvolutionExpander.cpp
  lib/Transforms/Scalar/IndVarSimplify.cpp
  test/Transforms/IndVarSimplify/lftr-udiv-tripcount.ll

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