[SCEV] Smart range calculation for SCEVUnknown Phis

The range of SCEVUnknown Phi which merges values `X1, X2, ..., XN`
can be evaluated as `U(Range(X1), Range(X2), ..., Range(XN))`.

Reviewed By: sanjoy
Differential Revision: https://reviews.llvm.org/D43810


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@326418 91177308-0d34-0410-b5e6-96231b3b80d8
5 files changed