[PATCH] Fixed several correctness issues in SeparateConstOffsetFromGEP

[Jingyue Wu]

Hi eliben,

Most of these issues are mishandling s/zext. 

Major issues:
1. When rebuilding new indices, s/zext should be distributed to sub-expressions. e.g., sext(a +nsw (b +nsw 5)) = sext(a) + sext(b) + 5 but not sext(a + b) + 5.
2. Function find should consider the bitwidth of the constant offset instead of always sign-extending it to i64. 

Added many tests to verify the changes we made.

http://reviews.llvm.org/D3985

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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