[LV] Support invariant addresses in speculation logic

Implement a TODO from rL371452, and handle loop invariant addresses in predicated blocks. If we can prove that the load is safe to speculate into the header, then we can avoid using a masked.load in favour of a normal load.

This is mostly about vectorization robustness. In the common case, it's generally expected that LICM/LoadStorePromotion would have eliminated such loads entirely.

Differential Revision: https://reviews.llvm.org/D67372

llvm-svn: 371745