RFC: Add option for interleaved loops, for loop vectorize.

The new pass manager uses LoopInterleaving to control interleaving in LoopVectorize after: https://reviews.llvm.org/D59723.
The legacy pass manager uses DisableUnrollLoops.

This is a proposal to have the legacy pass manager add a corresponding LoopsInterleaved field:

Clang change:

The intention is for the above two changes to not be a visible change for clang.

A separate clang flag that controls LoopsInterleaved can be introduced in follow-up patches.

Comments very welcome!

Thank you,