[LV] Sink scalar operands and merge regions repeatedly.

Merging regions can enable new sinking opportunities (e.g. if users of a
scalar value are moved from different VPBBs into the same VPBB). Sinking
in turn can also enable new merging opportunities (e.g. if a recipe
between to merge-able regions is moved.

To enable more sinking opportunities, repeat sinking & merging if
regions could be merged.

Also fix mergeReplicateRegions to return the correct Changed status.

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D139788