LoopVectorizer: If dependency checks fail try runtime checks

LoopVectorizer: If dependency checks fail try runtime checks

When a dependence check fails we can still try to vectorize loops with runtime
array bounds checks.

This helps linpack to vectorize a loop in dgefa. And we are back to 2x of the
scalar performance on a corei7-avx.

radar://15339680

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@193853 91177308-0d34-0410-b5e6-96231b3b80d8