More robust FILTER-LVAR through CASTs

 * IR1-conversion can insert casts between a combination and its
   arguments. Handle that case via principal-lvar{-use}.

 * Fixes a regression in b111015 (lp#1181684).
Free-er form FILTER-LVAR

The DUMMY argument can now be in any argument position.  Use that
in CUT-TO-WIDTH instead of ((lambda (...) ...) ...) hack.
Make ir1-convert-hairy-lambda safe for non-local exits.

The function it calls may throw a tag, locall-already-let-converted,
which will leave a partially initialized optional-dispatch structure
in new-functionals of the current component, which may cause problems
down the line.

Fixes lp#1180992.

Also add a test-case for f3a2cd.. "Add a stub for %other-pointer-p.".