We implement a transformation that propagates loads with a single
non-constant index in its deref path. Consider a load of the form
var[[a0][a1]...[i]...[an]], where ak are integral constants, and i is
an arbitrary non-constant node. If, for all j, the following holds:
var[[a0][a1]...[j]...[an]] = x[[c0*j + d0][c1*j + d1]...[cm*j + dm]],
where ck, dk are constants, then we can replace the load with
x[[c0*i + d0]...[cm*i + dm]]. This pass is implemented by
copy_propagation_replace_with_deref().
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