Constant propagation. Also detects use of variable before any possible def.
Do copy propagation. Copy propagation elems are of the form OPvar=OPvar, and they are in go.expnod[].
Mark all dead variables. Only worry about register candidates. Compute live ranges for register candidates. Be careful not to compute live ranges for members of structures (CLMOS).
Remove side effect of assignment elem.
Find e in Elemdata list.
Find all the reaching defs of OPvar e.
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Other data flow analysis based optimizations.