[LV] Use speculatability within entire loop to avoid strided load predication

[LV] Use speculatability within entire loop to avoid strided load predication

Use existing functionality for identifying total access size by strided
loads. If we can speculate the load across all vector iterations, we can
avoid predication for these strided loads (or masked gathers in
architectures which support it).

Differential Revision: https://reviews.llvm.org/D145616