Improve definition-use chain simplification for CSE

[fennecJ]

The current DU-chain implementation of #156 doesn't achieve the expected level of simplification for CSE. Since the variable is in SSA form and we're only interested in the last assignment for CSE, perhaps we can focus solely on du-chain.tail which record last assignment for a var?

[fennecJ]

I just realized that I mistakenly confused use-chain and du-chain. The use-chain tracks the series of instructions that use a particular variable, so iterating over it is appropriate. My earlier suggestion to focus solely on the tail was incorrect, as the use-chain involves multiple uses, not just the last assignment.

Additionally, the newer implementation actually simplifies the CSE flow by iterating only through the instructions that use the variable, rather than through all instructions in the basic block.

Apologies for any confusion or time wasted due to my mistake.