[PATCH] [SeparateConstOffsetFromGEP] inbounds zext => sext for better splitting

[Jingyue Wu]

Hi eliben, meheff,

For each array index of GEP in the form of zext(a), convert it to sext(a)
if we can prove zext(a) <= max signed value of typeof(a). The conversion
helps to split zext(x + y) into sext(x) + sext(y).

Depends on D4056

http://reviews.llvm.org/D4060

Files:
  lib/Transforms/Scalar/SeparateConstOffsetFromGEP.cpp
  test/Transforms/SeparateConstOffsetFromGEP/NVPTX/split-gep-and-gvn.ll
  test/Transforms/SeparateConstOffsetFromGEP/NVPTX/split-gep.ll
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