[LoopIdiom] let the pass deal with runtime memset size

The current LIR does not deal with runtime-determined memset-size. This patch
utilizes SCEV and check if the PointerStrideSCEV and the MemsetSizeSCEV are equal.
Before comparison the pass would try to fold the expression that is already
protected by the loop guard.

Testcase file `memset-runtime.ll`, `memset-runtime-debug.ll` added.

This patch deals with proper loop-idiom. Proceeding patch wants to deal with SCEV-s
that are inequal after folding with the loop guards.

Reviewed By: lebedev.ri, Whitney

Differential Revision: https://reviews.llvm.org/D107353