swift234

Improve dead store elimination compilation time.

If we know a function is not a one iteration function which means its
its BBWriteSetIn and BBWriteSetOut have been computed and converged,
and a basic block does not even have StoreInsts, there is no point
in processing every instruction in the last iteration of the data flow
again as no store will be eliminated.

We can simply skip the basic block and rely on the converged BBWriteSetIn
to process its predecessors.

Compilation time improvement: 1.7% to 1.5% of overall compilation time.
on stdlib -O. This represents a 4.0% of all SILOptimzations(37.2%)

Existing tests ensure correctness.