[LoopPredication] Insert assumes of conditions of predicated guards

As LoopPredication performs non-equivalent transforms removing some
checks from loops, other passes may not be able to perform transforms
they'd be able to do if the checks were left in loops.

This patch makes LoopPredication insert assumes of the replaced
conditions either after a guard call or in the true block of
widenable condition branch.

Differential Revision: https://reviews.llvm.org/D135354