This sets the stage for D133017 by moving out the code that performs
VPlan based simplifications to a separate transform that takes the
chosen VF & UF as arguments.
The main advantage is that this transform runs before any changes to
the CFG are being made. This allows using SCEV without worrying about
making queries while the IR is in an incomplete state.
Note that this patch switches the reasoning to use SCEV, but still only
simplifies loops with constant trip counts. Using SCEV here is needed to
access the backedge taken count, because the trip count IR value has not
been created yet.
Reviewed By: Ayal
Differential Revision: https://reviews.llvm.org/D135017
Users of LCSSA may not expect non-phi uses when checking the uses
outside a loop, which may cause crashes. This is due to the fact that we
do not update uses in unreachable blocks.
To ensure all reachable uses outside the loop are phis, update uses in
unreachable blocks to use poison in dead code.
Fixes#57508.
