[PATCH] Fixing inst-combine not to drops nsw when combining adds into mul (PR19263)

[Dinesh Dwivedi]

I agree with the idea that these cases should get handled in SimplifyUsingDistributiveLaws and
I am working on generalized approach for that.

I have updated patch to just put following changes in record.
I have removed check for overflow because after analyzing few cases, I think we should not
disable transformation to pattern like this even if (2 + 32767)
  %b = mul i16 %a, 2
  %c = mul i16 %a, 32767
  %d = add i16 %b, %c
  ret i16 %d
as original case and transformed case will result in same output. When I pointed out case similar to
this, I thought that for some value of 'a', '(b+c)' might be positive but if we add constant which will become
negative due to overflow, result will mismatch.
I realized that as 'a' in integer, if a == 0, we will get 0 in any case and any value of a > 0, b and c will never
be less than 2 and 32767 respectively. so add result will always overflow. 

What do you think? Should be disable above transform.

I have added a TODO to fix transform happening in SimplifyUsingDistributiveLaws and dropping nsw.
I will update it soon so that all these pattern gets handle in SimplifyUsingDistributiveLaws.

http://reviews.llvm.org/D3799

Files:
  lib/Transforms/InstCombine/InstCombineAddSub.cpp
  test/Transforms/InstCombine/add2.ll
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