[LAA] Be more careful when evaluating AddRecs at symbolic max BTC. (#128061)

Evaluating AR at the symbolic max BTC may wrap and create an expression
that is less than the start of the AddRec due to wrapping (for example
consider MaxBTC = -2).

If that's the case, set ScEnd to -(EltSize + 1). ScEnd will get
incremented by EltSize before returning, so this effectively sets ScEnd
to unsigned max. Note that LAA separately checks that accesses cannot
not wrap (52ded672492,
https://github.com/llvm/llvm-project/pull/127543), so unsigned max
represents an upper bound.

When there is a computable backedge-taken count, we are guaranteed to
execute the number of iterations, and if any pointer would wrap it would
be UB (or the access will never be executed, so cannot alias). It
includes new tests from the previous discussion that show a case we wrap
with a BTC, but it is UB due to the pointer after the object wrapping
(in `evaluate-at-backedge-taken-count-wrapping.ll`)

When we have only a maximum backedge taken count, we instead try to use
dereferenceability information to determine if the pointer access must be in
bounds for the maximum backedge taken count.

PR: https://github.com/llvm/llvm-project/pull/128061

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