[LoopPredication] Rework assumes of widened conditions

Currently after widening br(WC && (c1 && c2)) we insert assume of
(c1 && c2) which is joined to WC by And operation.
But we are going to support more flexible form of widenable branches
where WC could be placed arbitrary in the expression tree, e.g:
br(c1 && (c2 && WC)).
In that case we won't have (c1 && c2) in the IR. So we need to add
explicit (c1 && c2) and then create an assumption of it.

Reviewed By: anna

Differential Revision: https://reviews.llvm.org/D157502