[PATCH] Improve Cost model for SLPVectorizer when we have a vector division by power of 2

[Karthik Bhat]

Hi All,
Updated patch as per Sanjay's comment and submitted as r216371.

I think the reason we have different cost tables for different targets is because it may not be possible to come up with a common upper bound which can be profitable in case of all targets. 
For e.g. sra/srl/add/sra sequence may be the upper bound for X86 but this may/may not be true for other targets.
One more advantage which i feel of having target specific cost table is that it is easily extendable in case a new intruction is added to support some feature.

I'm not that familiar with DAGCombiner as of now will have a look at it and get back on this.
Thanks and Regards
Karthik Bhat

http://reviews.llvm.org/D4971