[PATCH] Partially revert r210444 due to performance regression

[Jingyue Wu]

Hi meheff, eliben,

Converting outermost zext(a) to sext(a) causes worse code when the
computation of zext(a) could be reused. For example, after converting

... = array[zext(a)]
... = array[zext(a) + 1]

to

... = array[sext(a)]
... = array[zext(a) + 1],

the program computes sext(a), which is actually unnecessary. I added one
test in split-gep-and-gvn.ll to illustrate this scenario.

Also, with r211281 and r211084, we annotate more "nuw" tags to
computation involving CUDA intrinsics such as threadIdx.x. These
annotations help with splitting GEP a lot, rendering the benefit we get
from this reverted optimization only marginal.

http://reviews.llvm.org/D4542

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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