fix structure stack allocation for high-debug code

fix structure stack allocation for high-debug code

  * Allow values to flow through casts in good-for-dx analysis.

  * Let-convert main-entry points for already inlined optional
    dispatches.

  * Don't preserve single-use debug vars in system-lambdas, no
    matter what the policy says.

  * Don't add INDEFINITE-EXTENT declarations to hairy entries without
    &REST arguments.

  * SB-C::REST-CONVERSION optimization declaration was pretty
    pointless, take it out.

  * Test our DX stuff in high-debug code as well.