[SimpleLoopUnswitch] Preserve make.implicit in non-trivial unswitch if legal

We can preserve make.implicit metadata in the split block if it is
guaranteed that after following the branch we always reach the block
where processing of null case happens, which is equivalent to
"initial condition must execute if the loop is entered".

Differential Revision: https://reviews.llvm.org/D84925
Reviewed By: asbirlea