[LoopPeel] Fix LCSSA phi node invalidation
In the test case, the BECount of the second loop uses %load,
but we only have an LCSSA phi node for %add, so that is what
gets invalidated. Use the forgetLcssaPhiWithNewPredecessor()
API instead, which will invalidate the roots of the expression
instead.
Fixes https://github.com/llvm/llvm-project/issues/109333.
