The patch replace the overflow check in loop vectorization with the minimum loop...

The patch replace the overflow check in loop vectorization with the minimum loop iterations check.

The loop minimum iterations check below ensures the loop has enough trip count so the generated
vector loop will likely be executed, and it covers the overflow check.

Differential Revision: http://reviews.llvm.org/D12107.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@245952 91177308-0d34-0410-b5e6-96231b3b80d8