[SelectionDAG] Mark frame index as "aliased" at argument copy elison (#89712)

This is a fix for miscompiles reported in
  https://github.com/llvm/llvm-project/issues/89060

After argument copy elison the IR value for the eliminated alloca
is aliasing with the fixed stack object. This patch is making sure
that we mark the fixed stack object as being aliased with IR values
to avoid that for example schedulers are reordering accesses to
the fixed stack object. This could otherwise happen when there is a
mix of MemOperands refering the shared fixed stack slow via both
the IR value for the elided alloca, and via a fixed stack pseudo
source value (as would be the case when lowering the arguments).

(cherry picked from commit d8b253be)