[PATCH] Disable indvar widening if arithmetics on the wider type are more expensive

[Jingyue Wu]

Hi atrick, jholewinski, eliben, meheff,

IndVarSimplify should not widen an indvar if arithmetics on the wider
indvar are more expensive than those on the narrower indvar. For
instance, although NVPTX64 treats i64 as a legal type, an ADD on i64 is
twice as expensive as that on i32, because the hardware needs to
simulate a 64-bit integer using two 32-bit integers.

Split from D6188, and based on D6195 which adds NVPTXTargetTransformInfo.

Fixes PR21148.

http://reviews.llvm.org/D6196

Files:
  lib/Target/NVPTX/NVPTXTargetTransformInfo.cpp
  lib/Transforms/Scalar/IndVarSimplify.cpp
  test/Transforms/IndVarSimplify/no-widen-expensive.ll
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