The patch adopts and reuses the optimizations from the legacy backend.
The optimizations remove useless temporary variables,
statements and simplify generated JS code.
The optimizations can be disabled by `-Xoptimize-generated-js=false`.
Related to KT-51139
All tests in this commit _pass_ to document existing behavior, but
this will change in a separate commit as we reflect desired behavior
in the test expectations.
Alexander Udalov