LoopVectorizer: Clear all member data structures in RuntimeCheck.reset()

Clear all data structures when resetting the RuntimeCheck data structure.

No test case. This was exposed by an upcomming change.
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