Follow up on #1981 - TDCU copy propagation correctness

[siegfriedpammer]

Currently aggressive TDCU only works correctly, if no local variable is assigned in the aggregate as well as the body of the ILFunction. See Test9 of the SROA test suite.

There are two options we already have discussed:
original and copy are ILVariables

1. ensure that original.LoadCount == 1 && original.IsSingeDefinition && stloc copy(ldloc original) dominates all other uses of copy
2. or neither original nor copy is mutated (after initialization), so all uses see the same reference: original.IsSingleDefinition && copy.IsSingleDefinition && copy.StoreInstructions.Single().MatchLdLoc(original)

Find a way to make sure either condition holds before and after TDCU or find another applicable condition.