[PATCH] PR19958 wrong code at -O1 and above on x86_64-linux-gnu (InstCombine)

[suyog]

Hi Duncan, Nick, Rafael, Sanjay,

I have modified the code to calculate shift by Log2(Const2) - Log2(Const1) 
instead of Log2(Const2/Const1) to avoid expensive division operation as per Duncan's suggestion.
Also handled few more conditions for 0.
Added additional test cases as well.

(I am doing this for icmp eq/ne only for now, 
other icmp instructions are TODO item as they need some verification)

Can you please help in reviewing this modified patch.

Your suggestions/comments are most awaited. :)

Thanks,
Suyog

http://reviews.llvm.org/D4068

Files:
  lib/Transforms/InstCombine/InstCombineCompares.cpp
  test/Transforms/InstCombine/icmp.ll
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