Handle concrete values in CodeFolding (#7117)

CodeFolding previously only worked on blocks that did not produce
values. It worked on Ifs that produced values, but only by accident; the
logic for folding matching tails was not written to support tails
producing concrete values, but it happened to work for Ifs because
subsequent ReFinalize runs fixed all the incorrect types it produced.

Improve the power of the optimization by explicitly handling tails that
produce concrete values for both blocks and ifs. Now that the core logic
handles concrete values correctly, remove the unnecessary ReFinalize
run.

Also remove the separate optimization of Ifs with identical arms; this
optimization requires ReFinalize and is already performed by
OptimizeInstructions.

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