[SCEV] Avoid copying ConstantRange just to get the min/max value

Summary:
This patch changes getRange to getRangeRef and returns a reference to the ConstantRange object stored inside the DenseMap caches. We then take advantage of that to add new helper methods that can return min/max value of a signed or unsigned ConstantRange using that reference without first copying the ConstantRange.

getRangeRef calls itself recursively and I believe the reference return is fine for those calls.

I've left getSignedRange and getUnsignedRange returning a ConstantRange object so they will make a copy now. This is to ensure safety since the reference will be invalidated if the DenseMap changes.

I'm sure there are still more places that can take advantage of the reference and I'll submit future patches as I find them.

Reviewers: sanjoy, davide

Reviewed By: sanjoy

Subscribers: zzheng, llvm-commits, mzolotukhin

Differential Revision: https://reviews.llvm.org/D32978

llvm-svn: 306229